Question

why does the earth need to rotate and revolve

Answer 1

earth's rotation causes day and night. if it wouldn't rotate then one side of earth would have only day and hot and other side only dark and cold.so earth need to rotate.
earth's revolution causes change in season. if earth wouldn't revolve around the sun, all the places in earth would have almost fixed season.

Answer 2

if the earth did not rotate than the side of earth facing the sun will have day while the side of earth away from it will have night.
and if the eartrh did not revolve than there won't be any season and only one season all the time.