you have your terms mixed up, plumbers do not use bronze pipe fittings they use " brass " fittings
if i am running a copper water system then i use all copper fittings
but if you are talking about a hydronic heating system then i might use brass fittings
the main difference between the two is cost as the brass cost more
on the other hand if i was trying to finish a solder job and all i had was brass fittings then i would use them rather then taking time to go get copper fittings