Ergonomics. It adds the option of my broken old wrists to not have to hold themselves at whatever arbitrary angle the probably younger and certainly healthier designer decided wasn’t causing immediate pain to them.

So yeah, having specific accessibility needs can suck and for my specific brand of those the joycon are an amazing option.

Also, playing a game with your hands by your sides or under a blanket or whatever adds a TON to the sense of immersion.