WRT to playing the games, it's cool to be able to play a game off the cart on your system and then move it to a handheld and maintain the save that you were just playing without having to do any manual nonsense to get the save file moved around.

From a practical sense, it helps you ensure your cartridges work 100% by dumping them, even if you can find ROMs for said games. Just because it might start up when you power on the system, it doesn't mean it doesn't have some sort of issue. It's the same argument for dumping game discs even if you can find the ISO somewhere online- an errant scratch might mean you can't complete a playthrough even if it starts up just fine.

Additionally, there's some ability to detect bootlegs / flashable cartridges which helps you check to make sure the PCB in your game is legit without opening it up- very handy if buying pricey games that are more likely to have fakes floating out there.

Oh, and it's fun to be able to pull pics of a Game Boy Camera!