What does backup and restore do besides the obvious?

[raidencmc]

Does it delete hidden files from old apps or something else?

 

[cdtommy]

nothing

 

[Indoler]

Does it delete hidden files from old apps or something else?

No. It does what it says on the tin.
Unlike Facebook or Google+, etc.!

 

[sting7k]

Does it delete hidden files from old apps or something else?

On Android devices...nothing.

 

[raidencmc]

So how would someone use it? Back up then master reset then restore? Or backup get new phone and restore to new phone?

 

[Indoler]

So how would someone use it? Back up then master reset then restore? Or backup get new phone and restore to new phone?

Correct.
Whereby it cannot be stressed enough that for real data security the backup data set(s) should be stored on another, physically separate device like a PC or laptop, or in a cloud, etc. Ready to be restored to a new phone.

 

[hallux]

In the Android world, the 'backup' is a backup of basic settings (not the home screen layout), your WiFi connections and what apps you had installed (but not their data). This is backed up to Google and will restore to any new device you sign in to with your Google account.