Some companies (in the Netherlands) don't want to deal directly with freelancers, for legal reasons. Also, they don't want to bother looking for developers themselves, while at the same time they are not interested in providing developers with any permanent positions. Usually they use a middle man to find appropriate developers for a fixed fee. Such a middle man might be able to provide you with several clients you'd not be able to contact yourself directly.

Some headhunters don't mind dealing with freelancers either. Currently I'm freelancing through Computer Futures.