File syncing and computer backup are two very different features. It is important to not confuse the two.

Backup:

Takes a copy of the file and uploads it to our servers for safe keeping. You can get your files back at any time if you accidentally delete or lose a file by downloading from the Control Panel on the web app or restoring via the desktop application.

Sync:

Allows you to sync the same files between multiple computers, syncing mirrors a folder on one computer on another.

Whilst Zip Cloud allows you to sync between an unlimited number of computers your paid license only allows you to back up one machine. You are able to purchase an additional license for a second computer for only 29.95 a year.