1) If you apply to places like Goldman Sachs off the street, chances are your resume is not going to even get a look. Of course, if you know somebody on the inside, that's the best way to get in, but if you don't? Going through a recruiter who has a good relationship with a client is the next best way to get in.
2) A good recruiter will teach you how to write a resume. Yes, some unethical recruiters will tell you to write things you never did, etc.. but a good recruiter will tell you HOW to write it the right way, so HR people don't look it over. Sometimes as a developer, you never know..
I've had friends who got phone calls from HR: "It says here you're a PHP expert, but do you know HTML? HTML is a requirement for this job." My friend didn't put HTML on his resume because he assumed anyone would know PHP implies HTML. But not to HR people, who are generally clueless.. a good recruiter will teach you that.
3) A good recruiter will teach you how to behave at the interview and what to/what not to say. Again, not talking about lying. It's things like asking a lot of questions, displaying enthusiasm, what to answer to "How much money you want?" (hint: the correct answer is "Look, I'm very excited for this job and it looks like it's a great fit.. I'm sure you'll treat me fairly, so I'm open to hear what you have in mind"), etc.
4) If you develop a good relationship with a good recruiter, he will give you invaluable career advice. For example: did you know that most hiring manager don't want to hire someone whose longest assignment is < 1 year? They call these people "job-hoppers" and never want to deal with them. I had no idea about this, before recruiter told me ...
There's a lot more, but the point is: there are some bad recruiters, and some good recruiters. Bad recruiters are to be avoided.. but good recruiters will give you a LOT of value.
Disclaimer: couple of my friends are recruiters.