In order to backup the Ledger device, what you end up doing is really basically a clone of one on the other. The procedure to do so is described here: https://support.ledger.com/hc/en-us/articles/360005434914.

As to the purpose of having a backup device (over just storing the 24 word recovery phrase), aside from what has already been stated (convenience for a prompt usage in main device fails), you can also keep if offsite (i.e. someone else’s home, knowing that it is a safety measure, and that the devices pin number should protect it from somebody trying to access it’s protected content – unlike storing a plain 24 word recovery phrase at the same location). If you do this though, just remember to update the firmware once in a while to avoid potential issues later down the line.