While increased speed isn't justifiable for many subscribers, as streaming and web-browsing is mostly all they do, there are a few fairly common use cases where

While increased speed isn't justifiable for many subscribers, as streaming and web-browsing is mostly all they do, there are a few fairly common use cases where slow speeds cause frustration (game downloads, and software updates, for example). Once the capacity is installed, there's little cost in providing a high speed last-mile connection to a subscriber, versus a lower speed using the same hardware, yet the data transferred by a subscriber translates more directly to increased costs for the ISP. I expect that in the future, price differentiation based on speed may go away, and data transfer caps / metering become more commonplace. Data transfer contributes more directly to the ISP's costs, and offering subscribers a faster connection makes it easier to consume.