You do use them for the holo cards, sure, but you put your whole deck in them. After all, you are gonna need all your cards to have the same back, lest you get accused of marking your cards (and thus, cheating). You're also gonna want a playmat (a hard surface will warp your sleeves after a while) and a deck box (to put all the cards in your deck in a container that can take a reasonable beating from normal wear and tear). For this game, since everything is done by cards, you'll only need a die when it comes to game supplies (for determining who goes first in a quick and easy manner while also marking who is your Lord). By comparison, Yugioh needs a method to keep track of life (typically a calculator, and there are ones out there meant for this exact purpose), tokens, counters (maybe even different kinds, since there can be multiple kinds), along with a coin and die for card effects.
And with all that, you can tell I've been playing TCGs for years lol. Just figured people who haven't really played TCGs who want to play this one would like this kind of info, is all.