With the introduction of OS X Lion, Apple has fundamentally changed the way OS X is sold and distributed. Gone are the install DVDs; OS X is now available as a download from the Mac App Store.
How to Install, Reinstall, or Restore macOS Using Internet Recovery | A Comprehensive Guide 🔧
With the elimination of installation DVDs, Apple had to provide alternative methods for installing the operating system, repairing startup disks and system files, and reinstalling the operating system. All of these capabilities were previously available on installation DVDs.
Apple's solution was to include an installer in the OS X download that would not only install the operating system on your Mac, but also create a hidden volume on your startup drive called Recovery HD. This hidden volume contained a minimal version of OS X, sufficient to boot your Mac; it also included several utilities.
As you can see, the Recovery HD can do much more than just install the OS. It offers almost the same services that were on the older installation DVDs, just in a different location.
