Lots of people use editing services, through their education, in their private life and at work. Sometimes it is because the person writing needs a helping hand on a certain editorial problem. Other time is it because the writer does not know the target audience as well as the editor. Sometimes it is because the writer has a blind spot to their weaknesses. Even great book and screenplay writers will make mistakes that they are completely unaware of until they are pushed under their nose.
People experience editorial problems all the time
Students are big customers of academic editors. This is not because students are particularly bad at essay writing, or because their work is particularly weak. It is because the academic community is made up of rules that have existed and evolved over hundreds of years. There are a lot of rules and for somebody starting out in their academic career, there is a lot to remember. You can teach a student to drive, and he or she may pass his or her test. But the student is still going to make a few mistakes whilst driving, until he or she is used to driving. The same is true for academic conventions and rules.
Sometimes an editor knows your target audience better than you
Editors are sometimes more like selective snippers. They snip out the bits of your text that alter the flow, or snip out bits that will not appeal to the target audience. A writer may be very astute when it comes to course material, but may not yet know his or her target audience well enough to make an impact on them. An editor can alter your work to fit the expectations (and lusts) of your target audience.
Editors can highlight your weaknesses
Writers will often enjoy writing so much that they cannot see the sections where they messed up. This is why some of the greatest writers will leave their work for six months, come back to it, and tear up 75% of it. They enjoy writing so much that they cannot view their work objectively and with a cold eye. Many people however cannot afford to store their work for six months, and so will hire an editor to do it for them.