The thing about the XB6 is that it's not just a dumb modem; it's the hub for a whole suite of integrated connected home products and services that Comcast has developed under its Xfinity umbrella. The Ignite TV bundle (with Internet / TV / Home Phone) is just the starting point for what we might expect.
However, it goes beyond that. For example, the XB6 can also advertise secondary Wi-Fi networks. The XB6 can enable an SSID to alert field technicians to network problems, and service providers can also leverage the install base of XB6's to provide public Wi-Fi hotspots to their customers.
Some of us will find it frustrating that we don't have total control over these devices, have relatively few configuration options on the admin screen, and have very little technical documentation. There are no option settings to configure alternate DNS servers, no option to enable/disable AP Isolation, and limited control over the Firewall. (I also did some digging and found a bit of information as to what the "Prefer Private Connection" option in the Connected Devices section is for.)
However, others will appreciate the integrated suite of connected home services that are designed to work together and where everything can be controlled through a mobile app. In order to deliver this functionality and to prevent things from breaking, it also means that the XB6 is effectively a cloud-managed device with very few local configuration options.