I think that many people imagine an editor's job or a copy editor's job as primarily, I don't know, placing commas in the right places and removing apostrophes from plural nouns.
And there's a lot of that, true. But I find that most of the line-to-line editing I do is about making sure that things make sense—that a writer is truly saying what he or she means to say. Even if a sentence's context makes its meaning very clear, errors of sense or logic can harm the impression a piece makes (even if the reader doesn't know why).
Here's today's sentence as it came to me:
An undergraduate degree is expected to succeed in HR consulting.
And here's the rewrite:
An undergraduate degree is generally required for success in HR consulting.
This problem sentence points out a lesson I've included in the first day of any copyediting or grammar class I've taught: Make sure you know what your sentence's subject is, what the subject is doing, and what it's doing that thing to.