Plains Matter decks use cards that care about the number of Plains you control. Mostly white, these cards reward a mono-white or Plains-heavy mana base with removal, tokens, and bigger creatures.
Lay Down Arms exiles a creature for a single white mana once you control enough Plains, while Gather the White Lotus creates a token for each one. Armored Ascension gives a creature flying and +1/+1 for each Plains you control.