Nope, they don't just polish the manuscript, with your approval they improve your work by 100% or more. I just completed my final rounds of edits today. I lost over 4,000 words from the edits but its a better story for it. Things were moved around heavily, motives were made clearer and stronger. More feelings and thoughts have been added. "He said, she said" were removed because it was obvious who was speaking, same with the thoughts, "he thought".
I really never knew the hard work that gets put into the editing process of getting books published and I am so glad I was able to learn from it. Seriously, editing is a much needed process before and after signing the contract. Now that I really know how it works, I can go back and edit my work better before sending it out for review by publishers or agents. The more you can do before it goes out, the less work you have to do afterwards!
Amber