Actually, I have no clue as to why Microsoft does this.
it does not include your personal files, just the windows and all subfolders underneath it.
It's basically useless since your already saved your documents, etc.
I delete it after my initial install.
Well, it did save my personal files. Everything I have branches from my Documents folder, and that was automatically backed up by the installation. I think it might have made the installation take lots longer, too.