Reasons why babies kick their legs?

Just for fun: Children do lots of things just for the fun of it.

Practice/tone: Babies have random movements that are part of the bodies needed tone maintenance. Long before baby will use legs to walk, s/he must flex/extend the muscle groups or they would shrink. Kicking is part of this plan, as are the random movement of arms .Over time the kid begins to enjoy the activity and start to have some control over the movements.