These sit between the "smartphone" and the untrusted, e.g., public, wifi router, acting as a user-controlled gateway.
The user makes a one-time change to the settings on their smartphone to use the user-controlled gateway. This enables blocking ads and other unwanted outgoing connections without having to be physically at home or work, i.e. in a location where the user can "trust" the network.
The pocket-sized router runs open source software chosen and installed by the user. Users have a generous choice of operating systems, from Plan9 to BSD to Linux, as they do for the RPi. Baseband is either absent or physically disabled.
The main advantage I see to leveraging old "phones" for this is the power supply.
While I have seen small form factor routers made for travel, they are generally not rechargeable.