Another use is to prevent accidents. Suppose you have multiple networks. For example, at work we have separate networks for machines that handle different kinds of sensitive data, with each network having the appropriate incoming and outgoing access rules to protect the kind of sensitive data on that network.
In such a situation, MAC whitelisting can protect against someone accidentally plugging a machine into the wrong network. Someone goofs and plugs a machine with customer financial data into a network that has unrestricted outside internet access? It's not on the whitelist for that network, doesn't get an IP address, and they quickly notice something is wrong.
In a home WiFi setting, MAC whitelisting could be used for partial sharing of your WiFi with people who don't know much about networking. Maybe I want to allow my neighbor, who has no idea what a MAC address is, to use my WiFi with his phone, but not to use it with his other devices. I could give him the access info and then use MAC whitelisting to just allow his phone on.
Finally, even if MAC whitelisting is not being used it would still be nice to be able to find out the Pi's MAC address easily, so that you can assign it a static IP address on your WiFi network.
Yes, you can find it out by letting it get a dynamic address on your network, going into the router settings and looking at the device table and figuring out which unexplained device is the Pi, and noting the MAC address.
Then you can configure the router to assign that MAC address a static IP address. In a world where consumer router firmware didn't often suck that would be fine. I've run into routers that once they have assigned a dynamic IP to a MAC address make it difficult to assign it a static IP address.
It's a lot easier if you can do the static IP assignment in the router before the Pi has ever connected to that network.
