What's the function of insult in Stellaris?

Mechanically, insult is at the moment nothing more than a temporary -100 opinion modifier, which may possibly goad an empire into attacking you, but it's unlikely to work unless it's a fallen empire.

Roleplay wise, it is of course a method to show somebody you really don't like them.

The function of insult was greater in other Paradox games, so I suspect it will be expanded upon in later patches/dlc (e.g. in EU4 it gives a casus belli and provides power projection)

When at war, with a low rep, issuing a ‘insult’ command will promp the enemy ships into combat. Unsure if it works at all times, but I’m finding the pattern to be the same, insult a warring faction that -for a moments time- is not attacking and they will say something like ‘now you’ve done it’ and send all of their ships to bombard you.