When I was being taught how to provide direction for creative people at Oglivy & Mather, they said that a narrowly focused objective was crucial to a campaign's success. 

I'm proud to say that I actually heard Norman Berry say "Give me the freedom of a tightly defined strategy." He said it often. 

Wired: Need to create? Get a constraint, 2011-Nov-13, by Jonah Lehrer (via DailyGood)

The larger lesson is that the brain is a neural tangle of near infinite possibility, which means that it spends a lot of time and energy choosing what not to notice. As a result, creativity is traded away for efficiency; we think in literal prose, not symbolist poetry. And this is why constraints are so important: It’s not until we encounter an unexpected hindrance – a challenge we can’t easily resolve – that the chains of cognition are loosened, giving us newfound access to the weird connections simmering in the unconscious.