There's a direct relationship between an organization's marketing strategy and its web site and the place we describe that relationship and begin to work out how we'll successfully achieve it is the creative brief.

Greg Storey in his ALA article, never get involved in a land war in asia (or build a website for no reason), relates a real-life example of just why there needs to be congruence between strategy and a creative brief. If there's no tangible reason and outcome expected of a site, how can we make decisions regarding how we'll execute the project?

The website assignment you each face calls for a creative brief. In this brief you will need to identify the rationale for the site's existence and then explain how you will achieve those stated goals. Yes, you'll need to tell us something about your approach in terms of the site's creative material, but without a foundation based on achievable and realistic goals, you'll just be applying so much chrome to base metal.

Storey's article is a great place to begin your understanding of what makes a good creative brief and why they're important. He quotes David Siegel,

A good web strategy fits in with the overall business strategy. It’s usually best to start with a focused, service oriented site and keep expanding from there. If you define the audience as “all teenagers” or “all people surfing from noon to 1 P.M.,” you will have to launch something the size of CNET to be successful.

— Greg Storey, never get involved in a land war in asia (or build a website for no reason)

A well-crafted creative brief begins by telling us the objective of the project in clear, precise language. It then tells us how we will achieve that objective, and how each element will contribute to meeting those objectives.

As related by Storey, there are times when you'll start this process and recognize that there's not a good enough reason (or business rationale) for doing the project—often because a more suitable solution presents itself as part of the discovery process. This is a good thing as it will meet our objective of providing good value to our clients/employers.

Other reading on this topic

Walter Stevenson, Designing through the storm
Wikipedia, Creative Brief definition
Canadian Market, Decline and Rise... (ad agency perspective)
And...
As great overall site with insight into the process of developing persuasive, successful projects, CBC Radio's Terry O'Reilly's Age of Persuasion.