Why?
Besides being incredibly interesting people, their sole purpose in life (as a recruiter) is to find you a job at the highest salary possible.
Why? Their commission, and their company's profitability, is wholly dependent on it.
This is important. They not only know how much you're worth in your given market, but because they are salespeople, they are likely connected to recruiters all around the country (US) that can provide you with that information in any market. They will also attempt to keep a long-running relationship with you, as they know that, eventually, you will be in the position to buy their services and are more likely to get that sale if they help you out now.
They literally benefit on both sides of the fence.
Recruiters helped me get to that coveted six figure land in leas than two years after graduating school. They helped me land jobs at some of the best companies in the country. They've taken me out on so many free lunches. They're great.
Now, I don't like all recruiters.
Robot recruiters are pretty useless (the ones recruiting from India for massive orgs that can afford to waste money). They collect your resume and go poof. Avoid them.
In-company recruiters are nice people, but, unfortunately, have the exact opposite motive: to hire as many people as possible as cheaply as possible. They will say whatever they need to say to get your body into a seat. They are the kinds of salespeople I don't like working with, since they work purely on volume and don't have time to get to know you.
This is how I got screwed by Google and regressed in my salary curve. (Mind you, the decision to work there was mine. I mean, who doesn't think about working at a place like that?)
So I usually go around them by messaging managers on LinkedIn and establishing rapport that way. By going this route, those recruiters become a formality instead of a gate, and you are way more likely to get paid at or over market. They are also much nicer to work with, since all they need to do to get their checkmark is move the process along and get your offer signed.
That is how I recovered from my mistake at Google and had one of my best working experiences to date.