What you want to make is a Mountain Lion Install USB thumb drive. That takes a file from the download and creates a USB stick that has all the Mountial Lion install files on it so you would NEVER have to download Mountain Lion again from the Apple servers. That is if you ever want to reinstall ML on your computer. Which is what you'll have to do if the hard drive fails.
You can also use it as a Boot device to run Disk Utility to correct disk problems or restore from a Time Machine backup or even just browse the net with Safari.
I have one for Lion and now one for Mountain Lion. Wouldn't be without them. I'm not really a fan of having to download a 3.5 or 4.7 GB file from the internet just to reinstall the OS. And what happens if you have no internet connection and you have to reinstall the OS. No internet mean no reinstall means no computer.