Flaps do 2 things: smaller flap deployments increase lift without increasing drag much; larger increase drag without increasing lift much (beyond that of using flaps at all).

You want to be going as slow as possible when you land (aircraft make poor racecars), and most aircraft call for some level of flaps on landing. Increasing lift lowers the stall speed, which is good, and increasing drag allows you to lose altitude without gaining speed. Without flaps they would have to land too fast.

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