An Internet Service Provider (ISP) has a need to authenticate users connecting using analog modems,...

Because of the broad support and the ubiquitous nature of the RADIUS protocol, it is often used by ISPs and enterprises to manage access to the Internet or internal networks, wireless networks, and integrated e-mail services. These networks may incorporate modems, DSL, access points, VPNs, network ports, web servers, etc.

References: https://en.wikipedia.org/wiki/RADIUS