I thought it may save space on the desktop ?
Yes it is possible to sync data to your iCloud and yet not backup your iPhone to iCloud. You can be signed in to your iCloud account for your iPhone to share the synced data (Contacts, iCloud Drive, etc.) and choose in Settings > [Your Name] > iCloud > iCloud Backups off. This will stop the back-ups to iCloud.
However, your strategy will misfire if you choose the alternative backup method, which is to backup the iPhone on your Mac. Instead of the backups being done on iCloud - which by-passes your Mac - they will be done on your computer's drive and consume space there; the opposite of what you desire.