For coaches with CBC couplings, there is an additional reason also. CBC couplings are also called automatic couplings. Basically, when the loco moves close enough to the coach, the couplings will be automatically locked. So, during reversal, if the detached loco is not moved forward, when the the loco is attached at the other end, the jerk can automatically couple the last coach to the detached loco. To avoid this, the detached loco has to be moved forward a bit.