Fetchlands are so good because Brainstorm is so good. If you Brainstorm without a fetchland, you put your two worst cards on top of your library, meaning you're

Fetchlands are so good because Brainstorm is so good. If you Brainstorm without a fetchland, you put your two worst cards on top of your library, meaning you're doomed to see those cards the next two times you draw. With a fetchland, you're no longer "Brainstorm-locked." Deathrite and Tarmogoyfs are considerations too, but they're not played nearly to the extent that Brainstorm is. Deck-thinning is another oft-cited use, but in practice, the extra land out of your deck is unlikely to give you a statistical edge unless you're shuffling and drawing a lot that turn. It's far better to save your fetches to shuffle away Brainstormed cards.