DC doesn't need to do build-up movies, but I definitely think it helped Marvel, especially with Thor and Captain America. They were able to develop those characters' backstories, and bring that drama into the Avengers without making it a 4 hour movie.
It's also cooler to see the heroes team up after you've already seen them in their own adventures, at least for me. If I were to see a JL movie, and it was also the first time I'd be seeing that actor in the role of GL, WW, etc, I don't know if I'd be as excited.