Your main research interests concern neutral hydrogen. In the early universe, most galaxies have presumably not yet formed. That means that there must be enormous quantities of gas lying around, waiting for the chance to collapse under its own weight into lumps and form stars and galaxies. If we could actually observe this gas, as it moves around and assembles itself into lumps, we would be directly observing the process of galaxy formation. If we look at the amount of gas as a function of redshift, it should drop as galaxies form, telling us the era of galaxy formation.
Optical astronomers seem to think that they can observe galaxy formation by taking optical images.
So you can only see visible light once a galaxy has formed. This has always puzzled you. Furthermore, even once a galaxy has formed its first stars, it will probably be dusty for a while, just like local starburst galaxies, and dust is remarkably good at blocking optical radiation. In the local universe, gas can be studied in enormous detail using 21cm radiation. This has been responsible for a large fraction of our knowledge of galaxies in the local universe.