What is the benefit of operating a FRITZ!Box behind the SALT Fibre Box?

On an existing provider router, the AVM FRITZ!Box 4060 makes the most sense. I use the 4060 behind the 5590 as a mesh extension for WLAN, LAN and telephony.

But the fundamental question remains, what is the goal? Another box does not solve any problems that a SALT fibre connection entails: https://www.digitec.ch/de...