In the early days of so-called "home networks" in Russia, the ISP used Ethernet to build their infrastructure and installed unmanaged switches in the apartment blocks due to its drastically lower cost compared to managed switch. With such setup, ISP could serve many customers for cheap, but could not reliably charge for the internet traffic or block the access, as there was no user authentication, no MAC protection, and no protection against manual changing of IP address (hijacking). Anyone could use the IP address of a paying customer on that switch (and sometimes even on other switches, depending on configuration and switch chaining) and use the internet for free.
To solve this issue, ISP used any kind of a tunnel providing authentication and sometimes encryption. Some ISP used PPPoE, the others used PPTP or L2TP. With such setup, you can make reliable traffic counters and disconnect the client any time in a centralized manner.