I mean... why wouldn't you do that?

In all seriousness let's look at it this way. A couple of things are converging here. First we can accept that streaming video is pretty much A Thing now. The average download speed for the US, right now, is about 36Mb/s. Keep in mind that's a rolling average so it's entirely probable that most people have around 25Mbs or less. Netflix SuperHD streams at 4.5-6Mb/s. So for a lot of people that's already a quarter of their available download speed at best. For single people or couples, whatever, that's probably fine. For families/households that's probably not fine since you'll have more people consuming bandwidth at once. So that's one part.

The other thing is 4k. Everyone has had a boner for 1080p for the last decade or so but that's gearing up to change with 4k. Now Netflix is streaming four times the bandwidth for their top quality content. Even if the average is 36Mb/s it's still JUST enough to stream 4k and not do much else with your connection. This is probably still a few years out since 4k TVs are still very expensive and why bother when you can get a giant ass 1080p TV for $500.

Comcast wants to be ready for this. It makes sense for a content provider to lay the groundwork for their upcoming 4k streams and what better way to do that than leverage the other thing you own, an ISP, to delivering faster speeds before the content is available.

You probably don't NEED gigabit to deal with that kind of video and still have the rest of your home network be usable. But if you can roll it out, why not? Especially when you can charge an arm and a leg for it.