To play the wide range of homebrew switch software created by the vast homebrew community the switch has, of course.

https://switchbrew.org/wiki/Homebrew_Applications

1. Then you don't have to carry a steam deck and a switch.

2. This way you can buy physical copies of games, keeping full control and the ability to resell them, while also carrying your current game library on an SD card.

Edit: I forgot modding! That's a huge use of emulation.

It's also worth noting that afaik dumping carts you own and playing them on an emulator is fine too. As long as you don't redistribute them.

So you don't have to travel with both your Switch and your Steamdeck. So you can play games at higher resolution and fps than the Switch. So people can make playthough and tips/tricks videos of Switch games without needing to buy a capture card. So you can use game mods without jailbreaking your switch.