I agree, I don't believe that you have
to cover it but it's a nice thing to do. It's sort of respectful to the deck. Not that the deck knows anything about it, I just like to look after the things I own.
I put my mobile phone in a pouch and with my tarot deck I like to keep it wrapped in a cloth in a bag.
Not for any mystical reason but I just think it's a nice way to protect the deck and it looks nice and makes it feel more special somehow.
Plus it's fun to pick out bags and cloths that I like.