John F, did the info given help you get a better understanding? I don't think it's something to get too hung up on. You said you didn't find good info on line. If you will look up the topic "setting gain stage" or "proper gain structure" you'll get a good idea of what these were originally designed for. A lot of it will relate to noise levels in your recordings. But, with the high quality of equipment components these days we are far less concerned about noise floor levels as we used to be. And, although getting a high VU reading level at the input while recording a track can be important that's not always a super high concern either as long as you are getting a high enough level to be able to mix the track with the other tracks without having to use plugins to boost it up. And, also high enough that you are not losing the quieter dynamics of a vocal or instrument but dropping off the input VU meter. And, of course, maxing out into the red on a digital recording is a bad thing.

Does the noise in your head bother me ?