Go next, basically.

If it's 4 minutes left and you are already down 10-0, or you have someone that left the game and you know will not come back, you might just want to GG and go to the next game.

About why people forfeit at 4 seconds left, well I'm no psychologist, but really some poeple dont want to sit even 4 more seconds like their life depends on it.

Same thing happens in LoL, people sometimes call forfeit when the game is on it's last push and will be over in 30 seconds.

There is no checkup 'there's only 10 seconds left so we block that option to you' so people use it even if the game is about to be over.

Some players feel that forfeiting lets them keep more of their dignity than losing does. If you forfeit with 4 seconds left, you admitted that the opponent was better, but still played what is essentially the entire game. If you had left earlier, you would be cutting short a fair match; if you had stuck until the end of the match, you wouldn't be communicating to the opponent that you recognize their victory.

I see this attitude more in 1v1, but I'm sure it applies to other modes as well.

Technical

Some forms of forfeiting are for technical reasons. For example, in a mobile game, there may be penalties for losing (or winning!) such as long, unskippable ads, and/or after-game animations. These can be avoided by restarting or quitting outright.

I can't recall a modern, competitive game where this is the case, but I've experienced it before. I've had games where matchmaking was long, so leaving ~5-10 seconds before everyone else got me in an earlier pool and helped reduce my matching time.

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Emotional

Personally, I feel like I retain control when I forfeit. That is, I deny the other team a true win and lose on my own terms—a positive action that loses, instead of my own lack of skill.

Since this question is pretty open-ended, I'll provide an alternative view: sometimes, a forfeit vote isn't as pragmatic as the other answers suggest, but is instead an example of non-verbal communication.

When the team that's forfeiting is not a pre-made party, voting to forfeit is a non-verbal way of saying "I don't want to play with you, anymore." Whether or not you also vote to forfeit is irrelevant to the non-verbal interaction.

Just like purposely shooting the ball into your own goal is a non-verbal way of saying "I hate playing with these teammates" or continuously demolishing a specific enemy player without doing much else is a non-verbal way of saying "I hate this player," this is a way for a player to show you how they feel about you.

There is a threshold for every game where it becomes impossible or really unrealistic to win anymore — say with 15-20 seconds left in a 3v3 while more than 1 goal behind.

If at that point all 3 players agree to forfeit, everybody can move on to a game they can still hope to win or learn from.

There will also be a point where a game is not fun any more for a player, for whatever reason: getting tired, getting lagged, finding that the team doesn't work well together... if a player is past this point, even only the last half minute of the game is not great.

In both cases the alternative to calling forfeit is to play something that is neither fun, nor interesting, and the only reason to do it is if you want to keep playing afterwards (so you can't take the 15-minute ban for leaving early). It's a chore.

In a 1:1 match, it can also be a matter of politeness: Playing an opponent who is not challenging at all for 5 minutes is not what most people enter a ranked match for. It can become a game of trying not to score, because scoring pauses the clock and makes the game drag on even longer. So a player might forfeit to let another player off the hook early if it starts to feel like a waste of time.

Forfeiting allows a new game to start

The short answer to why forfeit is to close the match without getting banned.

If you leave a ranked match without forfeiting, then you incur a 5 minute ban from matchmaking (both ranked and unranked).

Win based rewards

In my opinion, this is a large factor here that hasn't been addressed.

Earning ranked rewards in the game is solely based on wins at certain ranks. For example, if you want to earn Gold rewards, then you must win 10 times while ranked Gold or higher; however, you must do this for each tier before that as well. So that means you need to win 10 to get through Bronze rewards, win 10 to get through Silver rewards, and then you can win the 10 for Gold.

That is 30 wins, and considering that most matchmaking tries to pair you evenlyish, that means 60 games in the best case scenario. For people not taking Rocket League that seriously, for whatever reason they have (all are valid, this is just a game afterall), playing 60 games is a decent amount of effort. Tack on another 40 wins if you want to reach Grand Champ, and don't even get me started on how many games that is, because they aren't always at the right rank.

For this season I am 4/10 wins in Grand Champion, but have dropped to Champ 3 Div 3 at the moment, which means I need to climb back into Grand Champ to even begin to try to get the other 6 wins. It has not been easy.

There are some social norms to forfeiting

In most ranked matches, once you are losing by 3, it is probably a lost cause. 4 is very hard to come back from. So, depending on how much time is left, once you are down by 3, the general consensus and expectation is that it is fair to vote to forfeit without being toxic. Sometimes, you can be down 2 and really just play your heart out, but it becomes highly unlikely that you get a goal and then an equalizer in the last 30 seconds. By the last 10 seconds it is essentially not going to happen and sometimes people will forfeit then, even though as you note there isn't much time left.

Forfeit voting isn't consistent across league rankings

The overall behavior of forfeiting changes based on your league though. For example, even if you are getting Brazil'd (losing 7-1) in rank S (private pro lobbies) you will not see a forfeit. That is because they are training, sometimes messing around, but overall attempting to improve. Moving on to another match doesn't make as much sense.

Just below that is Grand Champ. It is excessively rare to see anyone leave a Grand Champ game before it finishes. There is always that hold out of hope to turn the match around, not to mention that most Grand Champ players are attempting to improve their play and get time in against strong opponents.

Champ.. This is where you start to see some of the issues commonly associated with "rage quitting". Missing an open net too many times, or whiffing saves too many times, especially when the opponent reaches more than a 3 goal lead, will almost always lead to a vote to forfeit and move on.

Diamond is just awful for this. Accidentally bump your teammate and they will vote to forfeit. Plat is a little better, as everyone there is mostly trying to get air balance down and are not entirely concerned about winning or losing. Gold is where most of the game is played, statistically speaking. It is an overall A for Effort space. Silver and Bronze are just a roll of the dice.

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Source: a lot of personal experience in ranked (for example)

Personally I have not seen a forfeit in the last 4 seconds when there was any chance for a change in outcome of the game, i.e., we are down by one goal and there is a chance of us taking control of the ball in the remaining time. That said, I generally don't see the benefit of forfeiting when there are less than 10 seconds left, specifically in terms of time saved.
I think the rationale behind such a forfeit would be to vent out frustration, or if you have got some new IRL task that you need to do quickly - answering the door/phone for example. Another motive might be that if you leave quick enough, you will not appear on the scoreboard at the end of the game, so your opponents don't see your rank.