While the people who pay the bills think that's the only thing that matters, your ability to confidently market yourself as providing perceived value plays a big part in your payment or salary negotiations, regardless of the actual value you provide.
Unfortunately I, like many others in our field, suck at this. Strangely, I've always found it easier to to market the work I do than to learn to market my own value. I can't even write a short bio without feeling like I'm stepping out of line.