It would reduce dwell time in two ways. Firstly, it would allow the train to enter the station faster instead of coming to a crawl (to ensure no one is on the tracks). Secondly, because of the doors, people would know where to line up to prepare to board the train, instead of standing where a door could potentially open and preventing people from exiting the train.
Personally I think the benefits might be a bit limited. Montreal has markings on the platform to let people know where the doors will stop, and that doesn't seem to stop oblivious people from standing right on top of the big yellow arrow pointing right at them, indicating the flow of passengers that they are about to block.