Sometimes, the agency brief is not the only brief. Sometimes, the client writes their own brief first. And then the agency writes a brief based on that (called a 'reverse brief').
This system, quite frankly, sucks balls.
Because when you have two briefs, the potential for confusion is immense.
I remember one occasion at a previous agency, when the most senior client had a strong view on what their next ad should be about. This was translated by their team into a client brief. Which was then translated by our team into a reverse brief. The proposition our creatives got was 'fun'. We duly wrote ads to fun. But when the work got back to the senior client, it had apparently wildly diverged from what was wanted - they had asked for ads about 'great service.'
Again I quote Mark Fitzloff, Global ECD of W&K, who said in a recent panel discussion: "Everyone between the CD and the most senior client is playing a game of telephone." ('Telephone' in some countries is called 'Chinese Whispers'). At first I thought this was a tad arrogant. But maybe he's right...
Having a great strategy is a great advantage in the creation of advertising. But even the best strategy will be unhelpful if it's one of two.
Maybe we have to accept that sometimes the client has their brief, and that's what they want answered. Planning was invented in an age when clients walked in with a product and not much else. It was up to the agency to decide how to sell that product. Nowadays, the client often has their own insights manager, has done their own research, and has their own view on what they want to say. What they want the agency to do, is to say that well. Not write another brief.