As someone once described in a post from a couple of years ago, running as root is like owning a giant robot suit, which allows you powers that ordinary humans can only dream about. Once you put on that giant robot suit, you can do anything you want, and no one can stop you.
Most of the time, wearing the giant robot suit seems really cool, but the problem is that every once in a while, people make mistakes, regardless of what they happen to be wearing. Running the wrong command as root, or accidentally moving/deleting certain files or directories can destroy your system, to the point where the only way to get your computer functioning agains is to reinstall the OS.
Don't run as root, except in specific, limited situations such as upgrading the kernel or installing new packages.