"Now as for some others he wrote included in his series...such as using the thumb on black notes in the chromatic scale...I think I understand his motive..not sure."
Yeah, these are the kind of things where it gets interesting. There are sometimes direct applications for doing this in real music, but it's basically about getting the hand into a lot of different positions and learning to be able to function normally when squashed together and playing at the back of the keys. There are other issues you could list, but I'd put those as the main ones. Actually, even here it's still basically a case where "it is what it is". It's not quite so specifically as with scales or chords- but it makes you put your hand in closed positions and forces you to play between black keys. That's basically what it's for.
"Remarkably there is a series of exercises written by Terrence Rust, in THIS century (21) that have NO explanation whatsover..as to what to expect by doing them."
Sounds unusual. One thing though- do we necessarily need to know what an exercise if for? If we practise them right, we ought to find out what the benefits are through doing them. You could argue that explanation of what you need to think about that is specific to the exercise is the more important thing (as well as proper understanding of how to move in general). For example, there are a lot of things I did wrong while trying the overholding exercises, in the past. I think I know how to use them now. When I practised them wrong I did more harm than good. When I discovered how to approach them healthily, the feel for what resulted was a lot more vivid than any description on paper could be. The description would likely talk about "independence" but it's a very different kind of independence than anything gained though running five finger exercises etc. There were very specific physical issues I had to be aware of to get that, however. I think they would be hard to convey in a short preface.