Why are $21\,{\rm cm}$ line observations more useful than observations of other emission lines?

Why are observations of the $21\,{\rm cm}$ line more useful that emission from other lines? Is it only because of its abundance in the whole Universe, or other reasons as well?

I only know one particular reason: The transition responsible for the H1 line is highly forbidden and shows an extreme lifetime ($10^7$ years), so the absorption rate in interstellar clouds, which can be very opaque for every other radiation, is very small. Looking at the H1 line allows you to see objects which are for example hidden behind dust clouds that absorb a lot of the radiation of the object behind.