In a nutshell, “purgeable” storage is hard-drive space the Mac can make available in an emergency — by automatically deleting files that you can always download again. For example, if you have the purge setting turned on and fill up your internal drive, leaving the system no space to work, the Mac will dump purchased iTunes videos. However, you need to turn on the Optimize Mac Storage setting to see the amount of space you regain from files the Mac considers worthy of purging in a pinch. The Optimize Storage option is also on the list. When you click the Optimize button in the box, files like old email attachments and iTunes video files you have already watched will be automatically removed from the drive if the Mac suddenly needs more storage space.
Source: New York Times January 11, 2017 17:36 UTC