For your case not much. But something that's important is much like the achievements you got, what you do in this game after beating it once is self imposed. And that will be the main driving force for anyone that plays this game after beating it. I come back to the game after getting the platinum trophy for the challenge runs. That might not be something you're into, but that's okay. Because a level below that are people who don't see a point to achievement hunting. The friend that got me into this game, traded his copy in, and was surprised to see me go into new game plus after beating it.

Now if you're asking why people are replaying this in comparison to other souls games, I don't really have an answer. I can understand what people mean by saying sekiro is the least replayable fromsoft game, but at the end of the day (in my opinion at least) it has the best combat out of all of them.