Genuinely curious, don't know much about networking.
Putting your router behind the DOCSIS modem lets you firewall the modem the same way you'd firewall the Internet at large - that is, an attacker who compromises the modem wins the ability to specifically monitor your traffic, but does not immediately gain free access to your local network.
For example, with DOCSIS 2 modems, you could spoof the MAC address and make some config changes and get anonymous internet access at the highest service tier.
Because of that TV heritage and the way they grew (on a town by town franchise basis), cable networks were usually a patchwork of really shitty networks up until fairly recently. My (limited) understanding is that on relatively modern cable systems, there is fiber connectivity to the local nodes, and then coax from that device to the homes in the area.
DSL is a switched network of sorts, and provisioning happens on the switch in the CO. Ditto for fiber.
