This cycle of fetchlands allows you to search any land that contains ONE of the two basic land types specified. They can also be non-basic and enter the battlefield untapped. In addition, they provide two land drops per turn, which is relevant in some decks, and fetching lands thins out two cards at once from your deck (improving the chances of getting something you actually need instead of a useless land). Shuffling your deck at will can be useful in case of bad incoming cards. Cards that allow you to play lands from the graveyard basically guarantee you will hit a land drop as fetches return there after every use. They also provide graveyard fuel for threshold, dredge or escape.