This keeps the host you've targeted so busy echoing back your pings that it can do little else. It also puts a heavy load on the network. Hackers with primitive skill levels will sometimes get together and use several of their computers at once to simultaneously ping some victim's Internet host computer. This will generally keep the victim's computer too busy to do anything else. It may even crash.
Flood pinging a computer is extremely rude.
Do this to a serious hacker and you may need an identity transplant. If you are using some sort of Unix, your operating system will let you use your computer to do just about anything to itself that it can do to other computers. Connected to localhost. My ex-husband Keith Henson did that to the Church of Scientology. The COGs ftp-ed to 127.0.0.1 and discovered all their copyrighted scriptures.