Is it necessary to back up old iPhone once new phone is set to and data erased from old phone?

FL-23602 Said:

"Is it necessary to back up old iPhone once new phone is set to and data erased from old phone?"

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About Backing Up your iPhone:

The adjective I'd use is "ideal". Backing up is ideal when it comes to using devices. In doing so, you'll have some thing to restore your iPhone from, should anything go wrong after the backup.

If data is erased from the old phone, it is not possible to back up that phone unless you set it up again.