I had an interesting discussion recently with a creative director on  how a creative brief helps creativity. We agreed that it has to do with mastering the right balance between freedom and constraint, between direction and inspiration. I then came across a definition by artist Trey Speegle who says:

You have to set up the narrow parameters that you work in, and then within those, give yourself just enough room to be free and play.

Which is really what I try and do with my creative briefs; give the creative team narrow parameters, but enough freedom so they can play. Result a piece of work that answers the business requirement, but in a truly inspiring, fresh, way.