It used to be that publishers would have a "Reader" on staff.  In these days of rationalization and outsourcing, the job of reader is, as often as not, a freelance position.

So, why would you hire a professional reader? Well, at the most basic level, because you want to find out whether or not you actually have a story.

Yes, I know, of course you have a story. After all it's what you spent the last (enter number here) years of your life writing.

The thing is, that while you may well have a story to tell, is it what you put on paper? Does what you wrote make sense, follow a logical sequence?

Does the dialogue sparkle? Does the action show what is happening or is it just explained? Show, don't tell.

Does the hero's hair colour change in the sixth chapter? Did he really just screw a silencer on to his revolver? (This won't work because of the gap between the chamber and the barrel.)

Is the language appropriate for the time period of the story?

These things can stop a great story dead in its tracks. And yet, these points just scratch the surface.

Other things such as the length of sentences. The syntax and structure of the writing, repetitious wording and writing patterns are marked for correction.