With MacOS Sierra, you can sync your Desktop and Document folders to iCloud Drive, but if you are a Dropbox devotee, you can get the same functionality without adding to the number of cloud services you use. Dropbox offers the ability to sync folders between it and your Mac. Here's how to set it up.
First, install the Dropbox app on your Mac. When the app installs, it places an icon in the menu bar. Click on the icon and then click the gear icon in the lower-right corner of the Dropbox window and click Preferences.
In Preferences, click the Accounts tab and then the Change Settings button for Selective Sync. This opens up a list of all of your top-level folders in Dropbox. Check the folders you would like synced between Dropbox and your Mac. A synced folder will hold two copies of each file -- one in the cloud in Dropbox and one on your Mac.
Any folders you uncheck from Selective Sync will be removed from your Mac's drive but remain on Dropbox, keeping your files in the cloud while freeing up some space on your Mac.