The built-in backup and restore does not backup and restore everything or even most things. It will backup settings of some apps (and some it won't depending on how and where the app stores them), may or may not backup your stock browser bookmarks, contacts, system settings, etc. It's not something to depend on for an actual backup and restore. And it will not restore things like game scores or files created with apps at all. It's very basic and under certain circumstances will not even work at all. It's more just a 'better than nothing' solution.
I disable mine, but some others use it. It's up to you. If you are going to use one of the backup apps there's no need for it and it will just take more time to sync your device with it turned on. But some people like to have it as a secondary backup. I manually backup anything that is important to me, which takes more work and most people don't want to do it understandably, but that's why I have no need for it.