Why use backup if icloud is already enabled for every application iCloud Backup refers to Settings &gt; [name] &gt; iCloud &gt; iCloud Backup and iCloud sync refers to iCloud &gt; Apps using iCloud and you enable/disable iCloud for the respective applications. Have a look at this  for reference.
I found the image online.
I am not sure if that enable/disable for the apps (the ones displayed like Mail, Contacts, Calendars, etc) works only if the iCloud Backup is enabled.
Most of what iCloud does (other than iCloud Backup) is synchronize data between your various devices and the cloud. Synchronization is not a backup! Synchronization protects you in case a device is lost, fails, etc, but does nothing to protect against data being deleted or corrupted. If you delete or overwrite an important note on one of your devices, iCloud sync will promptly delete/overwrite the copy on the iCloud servers, and then do the same on all your other devices. In a situation like this, a backup would let you go back to before the problem happened, but sync copies the problem around.
(As Allan pointed out, it's best to have multiple &quot;redundant&quot; backups -- a sync service doesn't count as one of them.)