Re: Question concerning absorption.
The reason all those things are discarded in the first place is because the blood has to maintain a balance of all these things. The kidneys are monitoring and filtering the blood in order to maintain this balance. Excess of anything, even all the "good" things, gets extracted and stored up in the bladder.
But, there's a constant game of "supply & demand" going on as minute changes occur constantly due to dietary, physical, environmental, emotional, and mental factors.
Because of this, what the blood didn't "need" very much yesterday, or 12 hours ago, or even an hour ago, it may have more of a use for now.
Urine is composed of all these elements that are absorbed into the blood through the walls of the digestive organs, and the entire digestive process can take quite a while at times, so from the time of eating, to the time of those elements being passed through the urine, can be many hours difference ... definitely enough time for subtle changes to occur in the "supply & demand" game.
Reintroducing urine into the system allows the body another chance at making use of things it didn't especially need when they were first processed (which could be anywhere from, say, 30 minutes ago to many hours ago). And the big plus to this is that urine is already in a format that passes very easily through the system. It doesn't have to be digested and filtered of toxins all over again... so it gives many of the digestive and "filtration" organs a little bit of a breather, instead of giving them even more work to do. Of course, I'm sure they still do some minor filtering and processing, but nowhere near to the extent that "food" requires.
So anyway... sometimes the body might not take much back from the urine (as far as nutrients and all that), sometimes it might make use of quite a bit of it. It's all very situational... and in general it just helps to keep everything much better balanced. On top of this, regardless of whether many "nutrients" are used from it or not, the urine itself and other aspects of it are very good for rejuvinating the internal organs that it comes in contact with anyway!
I'm no expert but that's how I understand it anyway. Hope it helps answer your question ... and I'm sure there's prolly much better explanations out there somewhere :)