wrote

HD's channels are low-bitrate streams

The station has a certain digital HD bandwidth that they can utilize as they
wish. They can allocate the full HD bandwidth to a single channel for the
best sound. On FM that would result in a CD quality sound. Or they can divy
up the bandwidth into 2 or 3 channels for lesser quality sound in each
channel.

of the same repetitive programming

If a station chooses to transmit two (2) HD channels, and many do, the
second HD channel's programming (HD2) is unique while the first HD channel
is the superioir sounding digital version of the analog signal broadcast.

causing adjacent-channel interference

Digital is digital - there is no static, noise, interference or fading
whatsoever on HD radio.

and with only 60% the coverage of analog.

Using 1/100th the power of the equivelent analog's signal carrier. Thus a
power savings to the station.