Why use backup if icloud is already enabled for every application

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 "redundant" backups -- a sync service doesn't count as one of them.)

There’s an old adage in IT regarding backups - if it doesn’t exist in 3 different places, it doesn’t exist.

iClould backup is just one backup for your iOS device. Ok, but you only use your iOS device with iCloud. True, but backups get corrupted - all the time! Having a second backup ensures that there are two different places you can get to if something disasterous happens. Like your Internet goes down (no access to iCloud backup) and your iPhone crashed and you need to restore. Having a local backup would easily solve that problem.

Three places? Sure! You have a document on local on your iPhone, on your iCloud drive, backed up to iCloud makes three. Having it backed up locally makes four places to gain access to that document.

Backups are one of those things that you take for granted until you really need it. I’ve never seen anyone complain that they had too many backups when their hard drive crashed or device was stolen. I’ve only seen people in despair when they didn’t have any.