This is a quality of life improvement. When retrying a map a lot of times, you will want to keep the beat across retries. When retrying, the map will be started earlier(with a slight delay) so the beat will start synced to the previous play. This will not be possible if the bpm is different, or when the music gets stretched out from failing, so this will only apply in the case of quick retry. If this is done correctly, we wouldn't need the "3, 2, 1" countdown before a map, and the delay before starting the map could be reduced slightly, since the player knows when the next bar will start, and is ready for it.