There are many great reasons for breeding fish. Here are a few that I could think of:
1. Breeding and reproduction is part of the lifecycle of any living thing. Experiencing it while under our control gives us a greater respect for the life we are preserving.
2. While you may be able to sell (or trade) some fish with a LFS - many breeders have found it much more lucrative to get involved in their own reselling operations to provide livestock to fellow aquarists that is much higher quality than something a store might sell, because you know that your fish was raised by a dedicated and caring person. (SN1 and Severum Mama are my personal favorites).
3. In the case of rarer species or species that have harvesting restrictions - captive breeding is an effective way of allowing these fish (and corals / invertebrates) to remain in the aquarium hobby without having to cause continuing depletion of the wild population.