We could say, it’s for some simple code that only belongs to that level. But it’s a bit like saying ‘how long is a piece of string’? Just because something is there, doesn’t mean you have to use it. All of these things are options.
It is possible to write a totally great game, only using the level blueprint, and nothing else.
It’s also possible to write a game, without putting any code in the level blueprint, player controller, or game mode.
Don’t fall for the youtube ‘this is where this kind of code goes’ thing. It’s very often up to you to figure out a good system for how you want your game to work.