Why do I need to backup apps?

Apps don't backup to iCloud. Only their data (if you choose to). If you don't backup an app's data then if you need to re-download the app, then all your data within the app will be lost such as the level you are in a game, etc.

It's not mandatory and strictly up to you what apps are backed up and what aren't. This is what iCloud backs up -- What does iCloud back up? - Apple Support.

This should help with managing your iCloud storage -- Manage your iCloud storage - Apple Support

An iTunes backup actually back up a few more things, so it is better than an iCloud backup.

You should have a backup in case your phone data gets corrupted or something you need gets deleted. It gives you a way to recover your data.