The initial reason why parents paid for the wedding was for transactional purposes. The bride was being transferred to the groom's family/clan. This was customary for many cultures.
Nowadays, I look at it as a way to help my child get setup for married life. It's the last gesture of their singlehood. However, there is a limit. If they want to go beyond that price then they will have to come up with the rest of the money.