I think experience and results is what does it. You have to be ranked yourself for one, then you have to provide results.

Along with that is your competence of SEO. You get paid because you know what you are doing, there isn't a secret to it. You can do it yourself or use someone that has their own successful techniques.

My clients utilize me because I know what I'm doing and it makes it easier for them, especially if they are too busy which is the case most of the time.

When someone pays for SEO work, they are too busy to do it, non-technial, or have a product that needs faster results than what they can do for themselves.