Not just the speed of changing the wheel. With old-style separate shoes and blocks, there was a real risk that a clumsy fitting of the wheel into the forks could hit and dislodge the brake block from the shoe. I've done that a few times myself. That would have meant extra seconds putting it back before the wheel could be reinstalled. The little guides made sure that the wheel went centrally between the shoes. I always thought that they were some extra weight for not very much benefit, but if Campag used them ... well, fine by me.