Question

why a pace bowler in cricket runs from a long distance before before he bowls

Answer 1

A pace bowler run-up to increase his balling speed. This is a basic reason.

Answer 2

It is because running for a long distance brings the bowler's body and the ball in motion and thus when he releases the ball the ball has much greater momentum since it has gained the momentum from the running and also the rotation of hand which makes the ball to be bowled faster.