A brief saves the same explanations from being repeated to every new writer, designer or agency. It is the starting point of every plan and every draft.
A brief is only useful while it is current. Prices, opening hours and offers change, so it needs an owner and a date, and drafts should be checked against it before they go out.
Example
A gym's brief: members are busy parents, the voice is encouraging without pressure, the trial week is free and classes start at 6 am.