Would you elaborate on this? What are your examples? Mods are known to update submission titles (in accordance with the guidelines), but I'm unaware of any other content changes they make.
1. They detach comments from comment threads (to hide them?)
2. They boost some stories so that they remain on the home page for a longer time
3. They also remove some posts from the front page... (because they are duplicates, not "quality" content, etc...)
I could probably come up with more examples, but I'm sure that you get the idea...