I use a prologue to introduce my setting, provide some backstory, and hopefully set the tone for my writing. At least that's what I THINK I do.
Edit - That is what I think a good prologue is. I wrote the examples below because, despite the differences, I THINK I did a good job with my prologues.
MFGR - Its 3 chapters around 10k words, where my MC dies, shows up in Utopia - The home of all the gods, and introduces the reason why the gods are giving him a system and reincarnating him into different 'Realms'. It also alludes to some issues the MC may face later.
DEN - 1 chapter 3300 words, MC is introduced as an old man, who dives in front of a truck to save two teenagers. He is brought to a strange place that is reminiscent of a daytime television gameshow stage. He meets a higher being, finds out he was brought there by accident, then joins the game show because that is something he has never done before. He then has to walk through one of ten doors to begin his story, diving into some sci-fi, fantasy, or death world. (Arguably my best work so far)
TN - It is a 1000 word Monologue by my crazy badger god-like being, describing how he implants systems in random people, which are not systems, it is just him. It then touches on the fact that Earth is about to become a death world, then makes fun of GoTs, because why not. My next 4 chapters could also be considered prologue-like, as they are all about the events leading up to the Earth becoming a death world and the aftermath of some of the survivors... not to mention more from The Narrator, and his minder, The Commander.