Massaging the breasts/chest before a pumping session can help your milk flow easier when the pump is applied. Think of it as priming the tissues and letting your. brain and chest know what's about to happen.
This skill is so, so helpful for a lot of reasons, but it's linked here because hand expressing after a pumping session can very often = a little more milk expressed :) which can lead to more milk stored and be helpful when trying to increase supply. Remember, an "empty" breast cues the body to make more milk. Supply and demand people!