All the 3 major ISPs in my area require that you specify the MAC address of your external LAN interface. It is only possible to connect to Internet using this MAC address (spoofing also usually works).
What is this limitation for? I think it's to limit the possibilities for vandalism of other users of your local network segment (other clients of this ISP served by the same local switch) towards you, but I can't think of any possible attack.
For instance, without the MAC binding other users could issue IP packets with my IP address as source address, but they have no way to intercept the incoming packets directed to me -> they wouldn't know what to send and where to in order to perform a meaningful attack.