Buddha said, suppose you were shot by a poisoned arrow, but instead of letting the doctor remove it immediately, you first need to know who shot it, his age, his parents, his hometown, his favourite colour, his favourite food, what type of bow he used, what the bow was made of, what the arrows were made of, and what feathers were on the arrow. You'll be dead before you get your answers.
Likewise with advertising briefs. The agency receives a brief from the client, and instead of giving it straight to the creatives - the doctors in this metaphor - who will solve the problem (pull the arrow out), it instead spends months getting analysed to death in planning.
“Everybody has a plan until they get punched in the face”, said the sagacious Mike Tyson. We know we're going to (figuratively) get punched in the face by client feedback, so aren't we better off just jumping in the ring as soon as possible, to give ourselves the best chance to get it right?
The cynic in me is aware it's corporate procrastination to extract more money out of our clients, who are happy to oblige because of an irrational enchantment of the mere mention of the word 'strategy'.
But make no mistake, the singular reason top brands use agencies is for their creative ideas. The rest they can do themselves. So, just imagine how much better the ideas would be, if the people doing the actual ideas, got more time to work on them? What if the man in Buddha's teachings just let the doctor get on with his job?