Free Up Space on Your Mac with Optimized Storage
With macOS Sierra installed, your Mac can determine the files you no longer need or the files you don't need locally and clear up space. The example on stage took a system with only 20GB free and cleared up 150GB free on a 250GB system.
This deletes duplicates and obsolete files like caches, duplicate downloads, old installers and items that have been in trash for more than 30 days.
Other items that you don't use often, such as some documents, mail attachments, old photos and iTunes movies you've watched will be in the cloud. The files you use and an optimized version of the photo will stay on your May.
On OS X El Capitan, you need to make these adjustments on your own, and track down files to delete or move.