Granted, the use of Time Machine makes restoring from backup relatively easy, but if for some reason you need to repartition the hard drive or install a new one on your system, you will first need to install OS X again.
While convenient for those with fast broadband connections, sometimes you may find yourself in a location without fast Internet and would benefit from having a way to reinstall OS X from scratch. Instead, if you wish to reinstall OS X you simply reboot to the OS X Recovery HD and use the tools there to format your hard drive (should you choose to), then run the installer to download OS X from Apple's servers.
Since its move to online purchasing and distribution of its operating system software, Apple has no longer included gray recovery DVDs with its Mac systems.
