Correctness and accuracy. Snore!
Let me take care of the boring stuff. Grammar. Mechanics. Spelling. Blah! Just kidding, I love it. I'll make sure your work is correct and consistent.
Many people don't really know the definitions for the different types of editing. Copy editing refers to what most people think "editing" is. But, for the proper definition, let me put on my monocle and my loftiest voice: copy editing is to ensure correctness, accuracy, consistency, and completeness. It includes editing for grammar, spelling, punctuation, and usage.
I mean, while I am writing this, I am truly trying hard to add some pizzaz and keep you interested, but you probably find this stuff really boring. But that's the thing, I don't! So take advantage and pawn it off on me. I will take notes scrupulously and make sure that you always use the right mechanics, I will also make sure that the room number for Stacy's English class is the same in chapter 4 as it is in chapter 14. Or that your character's car is always a blue Toyota and never a green Mazda. Or that you always use an Oxford comma (or never do, for that matter). Maybe I am making sure that your dates are consistent or that your math adds up (just kidding, please don't give me math to do).
Anyway, I can't really sell that copy editing is going to be your favourite thing and that you'll be itching to get started. But that's kind of the point. Let me do it! I like it and I know what I am doing. I've been trained! My university says so!