If you plug in an antenna and scan for local channels, what used to be one analog TV channel on colloquially-named "channel 7" in the defined channel 7 frequency range will now be seen as "channel 7-1" which is often (but not always!) actually broadcast on the old frequency. During the ATSC transition, it was important to retain the branding and consumer memory of "channel 7" even when the RF transport frequency changed.

The transition opened the ability to have an arbitrary number of digital subchannels, 7-2, 7-3, etc etc. The total bitrate available on a given ATSC broadcast is fixed, but it can be partitioned among multiple MPEG streams. The typical setup is one (occasional two) HD channels and several SD channels. Obviously there's a balancing act between bitrate and visual quality for each subchannel. It's often used as a way to broadcast older (read: cheaper) syndicated SD content on which they can run more ads while keeping the main HD channel quality reasonably acceptable.

The people who watch it vote in numbers far out of proportion to their peers. Nobody would bother to capture the industry if it weren't worthwhile.

We've got internet, but no cable. So we still watch over-the-air TV, though it's mostly PBS at this point (which is non profit). Also listen to FM radio - mostly NPR, a jazz station and a classical station - all also non-profit.

It's not that people are literally watching these on over the air digital TV antenna systems, but that if you have a "Broadcast TV" station in a certain market, it becomes carried by default on cable systems, direct to home satellite satellite dish systems, and regional FTTH providers' last-mile cable-tv-equivalent products.

Look at for example how all the Seattle TV stations are carried on Comcast or Astound's networks.