macOS Sierra has a new feature called Optimized Storage. It helps you save space by storing your content in the cloud. It also sports tools to help you find and remove large or unneeded files.
When storage space is needed on your Mac, files, photos, movies, email attachments, and other files that you seldom use are stored in the cloud automatically via Optimized Storage. Each file stays right where you last saved it, and downloads when you open it. Files that you’ve used recently remain on your Mac, along with optimized versions of your photos. Follow these steps to implement Optimized Storage:
Click the Apple logo () in the top-left side of the screen, select About This Mac, then click the Storage tab. You'll see an overview of your free space and the space used by different categories of files, including apps, documents, and photos. Click the Details button on the right-hand side.
Click the Manage button to see recommendations for optimizing your storage. You'll see fewer options if some of them are already turned on.
Optimized Storage presents four different options: Store in iCloud, which moves photos and videos to iCloud to save room on your Mac; Optimize Storage, which removes previously-viewed movies and TV shows; Erase Trash Automatically, which automatically empties your Trash after a file has been there for 30 days; and Reduce Clutter, which lets you review and delete older documents — a bit like various third-party app cleaners.
By the way, you’ll only want to choose Store in iCloud if you’re paying Apple for extra iCloud storage (or plan to). The 5GB the company gives for free fill up VERY quickly.