It's a common axiom that teaching someone a topic helps deepen your own knowledge in said topic. For example, I regularly tutor people in how best to use Microsoft Excel for their jobs. As a result of this, my own mastery of the program has increased tenfold. The reason for that increase in mastery is because I'm forced to explain how to do things to someone else after I've learned how to do it myself. This double-use amplifies the original knowledge simply by virtue of using the topic a whole heck of a lot.
Editing a story that someone else wrote has the same effect. When you take the time to really look at someone's words and read through them for content, coherence of plot and story structure, or even for grammar/stylistic gaffes, you find that you're more aware of those errors in your own writing.
I've been editing full-time for about 5 years now, and for about 5 or 6 years between high school and college as well, and one of the key things that I've realized is that I can write much cleaner copy now than I ever could before. Editing has also allowed me exposure to a wider variety of writing styles, which has thus allowed me to cherry-pick as it were from the way certain people phrase certain things.
I've also become more aware of issues in my own writing since I've become a full-time editor. Specifically as it relates to news articles and press releases, but also with turns of phrase in my fiction as well. I don't edit fiction with the same regularity as I do (or did) news pieces and press releases, but the lessons can be similar. Inconsistency of tone, irregularity of sentence structure, grammar mistakes, and so on -- these can be learned by editing pretty much anything.
One of the most curious things I've noticed is that by becoming an editor, I'm more attentive to the way things are written no matter what I read. In novels, I can notice areas for improvement much more quickly than I ever could before. In news articles, I find myself re-casting sentences with an alacrity I previously never had. In press releases, I've become highly attuned to the stylistic differences between PR firms and can now format and write a press release surprisingly fast.
The end result of all this? Being an editor has made a significantly better writer than I ever could have been without that experience.
How has editing someone else's work improved your own?