mtg blue search your library for a card Rules Text: search, target, player's, library, exile, 21 cards reveal it Rules Text: search, your, library,

Agathas Soul Cauldron Targeted graveyard removal every turn for an initial cost of two is already extremely useful, and thats just the start of what Agatha can cook up with this cauldron

Norn's Choirmaster gives your Incubator tokens another buff with frequent proliferation

There are other similar mono-color rocks in this semi cycle, but these are the best

They serve to give your side of the board tempo by allowing your creatures to attack (removing defenders), giving you more time to set up the board (removing attackers/expensive creatures), and allow for combat tricks (saving your creatures after damage is on the stack