Guide to Card Sleeves
Card protectors, or sleeves, are perhaps the most common accessory for games. There are two main reasons for sleeving your games:
To protect the cards (kinda says so on the tin)
To increase the sense of quality, much like component upgrades
The protection part is especially important if the cards are of high value and/or gets shuffled a lot. Both are true with most collectable card games (CCGs), like Magic The Gathering – and this is why the sizes used for these games has the best selection. Shuffling with sleeved cards feels a lot better than unsleeved, so that affects both point 1 and 2. You can also get them with matte finish, to reduce glare.