You erased your startup disk, or are setting up a new startup disk.
Some tech pundits recommend a clean install of OS X when a new update has been released. Apple released OS X El Capitan on September 30, 2015. This can be a good opportunity to clean out the hard drive of accumulated and outdated system files in preparation for update, particularly with older computers.
Follow these links to Apple’s support pages:
If you need to reinstall OS X, you can use one of these methods:
It is widely reported that the download of El Capitan is very slow. One option if you are planning on upgrading multiple macs is to use the usb bootable installer process, linked above, so that you only carry out one download and then locally install each computer.