If you choose some folders to backup or sync from computer to your Dropbox account and they are shown in the Dropbox folder in "Finder" and uses double storage at computer. You can use "Selective Sync" to remove folders from the local drive after they have fully synced to Dropbox.
The Dropbox Selective Sync feature allows you to select only the folders which you want to be synced to your computer. It is the feature of the Dropbox Desktop Application. It gives you the control to ensure that you have only the files you need on any computer. You need to enable the 'Selective Sync' feature to save the space on your computer. To do so, download and install "Dropbox Desktop Application" on your computer. Click "Dropbox" icon located at the system tray and click "Gear" icon. Select "Preferences..." from the menu and click "Account" tab and then click 'Selective Sync' button. You will see the folders in your Dropbox folder. Check mark the folders that you want to sync to your computer and tap "Ok" button.