Install macOS Sierra If You're Low on Space
Is your Mac almost out of storage space? macOS Sierra is here to help with Optimized storage. If you have enough space in iCloud or if you upgrade to add space your Mac can automatically move the files you rarely use to iCloud so that you don't run out of space on your computer.
This will also help you get rid of duplicate and obsolete files that can clog up your computer with useless data that hogs important storage. Apple continues to optimize this feature.