Most of mac OS Sierra's storage features center on iCloud and they're a mess. As part of the setup process Sierra asks if you want to store your documents and desktop on iCloud.
If you use your Mac for business, I recommend rejecting that offer. Why? Because you'll never know when you'll need a file and - when you do - if you'll have Wi-Fi available to reach it. Plus, several copies is the only way to protect data that you care about.
You are much better off using a backup service, such as Backblaze, or a dedicated file sharing service such as Box or Dropbox, to store files that you want to access from multiple devices. In addition, keep a daily local backup - Time Machine works - to ensure that your data can survive multiple device failures.
Archive unneeded files to a couple of bus-powered portable drives. There are cheaper ways to go, but notebook drives are optimized for frequent power-ups and rough handling, good things for an archive.
The cool new feature
OK, so forget iCloud for now. The cool feature that helps you manage capacity use is buried in the About This Mac, not System Preferences.
Click on the Apple menu in the upper left hand corner of you screen, then click on About This Mac. In the small window that pops up, click on Storage, the center option in the menu bar, then the Manage... button.
The first time you do this a window will drop down labeled with the name of your drive, its capacity, and its free space. The window will look like this:
Per my advice above, you can skip most of these options, although the Empty Trash Automatically option is worthwhile for those who forget that chore. But on the left side is a list of major folders in order of the capacity they use.
Click on one of those folders, select File Browser and all files are listed in order of capacity use. This makes it easy to find the big files.
Hover the cursor over a file and you'll be give the option to either move it to trash, or see it in a Finder window. A fast and efficient method to trim a bulging drive.
I particularly like this feature because it replicates functionality that I've used for years in The Omni Group's free - and great - utility, OmniDiskSweeper. Apple's version benefits by following Omni's lead.
The Storage Bits take
One of these days, I sometimes hope, Apple will take storage and storage management seriously. It is, after all, the data in your Mac or iPhone that makes it yours. No data, no love.
But Apple continues to disappoint. It charges way too much for flash, Time Machine, and online capacity. Its replacement for the ancient HFS+ file system gives short shrift to data integrity. Time Machine's pretty UI conceals a deeply flawed architecture. And Apple's spotty record with online services hardly encourages trusting critical data to them.
So the new tools in the About This Mac storage manager are a welcome surprise. They give control to the user through a simple and logical UI. Casual users will be happy with Apple's automatic options in Sierra, because they are easy.
But business users need more, and this little tool is very helpful.
Courteous comments welcome, of course. While my desktop machine has 25TB of storage, my MacBook Air has only 121GB, so I manage capacity closely.