Im a magic player, and yes sleeves necessary to protect magic cards from damage and wear which will devalue a card's worth. Magic cards can be quite expensive individually, and the financial investment one puts into it should be protected.
As for tarot sleeves... the thought came across my mind a few times. With the size of tarot cards, it would make shuffling more awkward and take some time to get used to.
However tarot sleeves offers a new possibility. Many tarot decks are the same size, especially decks from the same publications. With sleeves, you can mix cards of several decks to get a unique combination art and feel. Mix vampires with bunnies. Mix traditional imagery with some really different art styles. You can have the swords suit for one deck and cups from another. Don't like how Death and Justice look in one deck but love the rest? Replace them with ones you like! There is some fun to be had with mismatching tarot decks, and sleeves provides the neutral card backing to do a proper reading. There are some tarot apps on phones by "The Fool's Dog" that provide this kind of experience, and its really is interesting, but this way, you cater the decks you want.
Also card sleeves an help someone use a deck they like everything about BUT the card back. Lets be honest, some card backs are ugly. Or they are not conducive of reversals (card backs that aren't mirrored designs) and you want to read reversals. Sleeves can open those possibilities for people.