First time: discovering the overall plot and getting to know the characters
Second time: getting to know the characters deeper as you can cross reference subtle personality quirks or language patterns they express early on with the developed character you already know from the last playthrough. Same thing with the game world or setting. You now have the context to understand all these refferences and forshadowing, you can see the personalities of the nonfamiliar characters when you first meet them and how they act towards you contrasted with their more developed friendly version later on, and being aware of more details about them get another dimension of understanding about them based on how they choose to hide or alter themselves. If the writing does not have these qualities its basically shit.
Third time: Having played through the second time and gained this understanding of the characters and world you can now play through a third time, this time knowing the entities in the game as their most complex and closest to true selves and examine their reactions and actions and motivations at each step along the way through the lense of knowing each stage of their own revealing to you related to how they appear. The deepest understanding
Literally no reason to replay a shit game with bad characters though
