Is a chaddi required?

Seriously how important is a brief in advertising. On one end people say think beyond the brief on the other the same demands one.What is the importance of a brief that's just a formality, that has no value addition from the brand servicing guy? When a creative guy works on a brand for say a year and a half, does he really require a brief? These insights are based on my experience in mid-size agencies as I have never worked in the top twenty.

I maybe wrong but then what if i am right.