I-formation in doubles tennis

Firstly, it can break the return rhythm of your opponents.

Secondly, the return player cannot return the ball cross-court, but only longline. This might force him to errors or a weak return.

Even if he tries to return cross-court, your partner at the net can hit the ball to the open side (cross-court).

Here is a translation of an explanation of the Australian- and the I-formation from a german tennis forum:

Australian lineup

Your opponents are returning heavily and you are having a hard time getting your serves through? In this case it might be worth considering switching to the Australian lineup.

In the Australian lineup, the server stands right next to the center marker (= the tip on the baseline). His partner is in the same half of the court, positioned next to the center line. In the following, the net player is responsible for the left side of the court, the server for the right. So it is exactly the other way around compared to the classic lineup.

The Australian lineup is an effective variation if you want your opponents to to force your opponents to make more longline returns. The server typically plays the ball through the middle or to the body. The return player is now forced to return longline if he wants to avoid the net player. This change often throws the returner out of rhythm, especially if he has been returning mainly cross-court. He may make more mistakes now than before.

With the Australian formation, the net player can also intervene by moving diagonally forward to the other half of the court and intercepting the return. This is often even advisable, because the server may not be able to move up quickly enough and would therefore be forced into a difficult volley from the half court.

From a geometric point of view, the Australian formation is slightly disadvantageous compared to the conventional formation. This is evident from the fact that the server has a longer run to cover the court. The advantage of the Australian variation is mainly the surprise effect. It reverses the normal pattern and can thus disrupt the rhythm of the opposing team.

I-Formation

The I formation differs from the Australian formation in that the server's partner kneels crouched over the service line. This variation is also designed to cross the return rhythm of the opponents. Since the net player can intercept returns hit crosscourt, the opposing team may feel compelled to play longline.

With the I-formation, your opponent may be forced to make riskier returns and commit more errors. For the I-formation to work, your partner at the net must be able to parry powerful returns with reflex volleys. The advantage is that the net player narrows the angles for the return because he is closer to the returner (similar to a soccer goalie moving toward the shooter). A disadvantage could be that with this set-up you give the opposing team more space to lob cross-court.

Translated with www.DeepL.com/Translator (free version)

EDIT: Here is the source: https://www.saitenforum.de/board/showthread.php?t=25488