Wow…you don’t want much do you?
Basically, 100% height will force the block element to expand to fill the available vertical space available to it regardless of the content.
It’s useful for making ‘full screen’ web pages, you know the ones where the footer isn’t halfway up the screen. There are other uses but did you have a more specific question?
I really don’t use it unless I have to…and I don’t have to often.