With no brake the prop will just windmill. With brake the prop will stop and help slow the plane down quicker. Some people use a very low idle for landing saying that it will act as a brake as opposed to letting the prop windmill. I didn't find this works on my setup and use the brake (low trim) on all but windy days. My throttle trim is a left sided slider making it easy for me to put on the brake or not.

Stuart