“We need a campaign.” It is a familiar starting point. It also leaves a lot for your agency to guess.
Are too few people finding you? Are the wrong people enquiring? Or are interested customers dropping out before they buy? Each problem calls for different work.
Imagine a business asking for more leads when its sales team already spends hours filtering unsuitable enquiries. More volume could make the problem more expensive. The brief should explain what needs to change, who is affected and what the business has already tried.
This is where a brief earns its keep as a thinking document. Writing it makes you examine the request before paying someone to execute it.
You do not need a perfect diagnosis. Share what you know and where you are uncertain. A useful agency will help investigate. Arriving with a clearer problem gives that conversation a better starting point.