Just like an author needs an editor, so do websites. Why? Because pretty much it is just words on the screen instead of words on a paper.
Now website has a lot more pieces though. Navigation, do all the links work, how does it flow all together? All these things need to be tested and thought of and it is best if you have someone else look at it because they don't know how it is suppose to look. So what might look like easy navigation to you (because you know where everything is) isn't to your customers.
Look at any good book that is published. They all have had an editor. People need their websites to go through the same process. If they are going to pay someone, they might as well pay you. And if you get good and can do a quality review in a short amount of time, then they can get it up and running faster which means more money for both of you.
If you want to you can start helping people here with their websites. People are always asking for help. Now I am not saying you should charge the people here, but it can help you gain experience and testimonies for other websites. DO NOT SPAM PEOPLE HERE!!!