The summer issue of Booz, Allen, Hamilton's Strategy + Business has some great stuff. One of the best is an article by Edward Lnadry, Andrew Tipping, and Jay Kumar, Growth Champions (registration required). In it, they give the best definition for the purpose of marketing that I have seen:
...today, they’d say it is the ability to grow a business, particularly in mature or established industries, or across an expanding base of potential customers. In an era of unlimited opportunities but constrained resources, the only marketing metric that matters is growth. Driving growth means stretching the traditional boundaries of the marketing function to encompass activities many companies don’t even think of as marketing &mdash yet.
Every company looking to accelerate its growth should keep that in mind.