Much of the free SEO advice that is available online is outdated. Sure, at some point that information might have been correct, but changes in the algorithm could make that information incorrect.
A professional SEO service on the other hand constantly monitors the changes in the algorithm so they can give the most accurate advice. This ensures that when you do hire an SEO service, that it will work for the current algorithm. Since an SEO service spends so much of its resources on finding out how to increase ranking in the algorithm, it will probably also be the first to know how the changes in the algorithm are affected. This means that you will be at an advantage over other websites that are relying on free online advice.