What does "Reinstall macOS" do, exactly?

//I was recently having some problems with iTunes, and decided to reinstall macOS Sierra. I held Command+R on startup and clicked on "Reinstall macOS" without erasing the hard drive in Disk Utility. Once the reinstall was complete, I started back up and all of my files and programs were still there, along with my settings, desktop wallpaper, and everything else. (The iTunes problems were fixed.)//
What exactly does this "Reinstall" feature do, if it doesn't remove programs and change settings to default?//
Here are answers on-line:- https://apple.stackexchange.com/questions/276766/what-does-reinstall-macos-do-exactly
How would YOU have answered the OP's question?