Not sure why people here are all arguing about why you would want to use discs, rather than the fact that the Steam Deck is a PC, of course you can absolutely used discs. All you need to do is plug in a USB disc drive, and it’s ready to go. I’ve installed a bunch of my older PC games from CD/DVD that way, and it works great. Even under Linux, applications like Lutris make installing Windows game discs pretty easy, and once they’re installed, you’re ready to go.