Yes, urine is a waste product, like oxygen is a waste product of photosynthesis. Since male sperm is pretty sticky, male mammals have to clean their sperm channel regularly, and they do so with the waste product urine. Birds use the waste product urine to clean their cloaca.
Indeed there are no nutrients in it. But it cleans. In ancient Rome, it was used where we would use soap.
Long distance walkers share a secret: urinate over your feet in the morning, then pull on your socks, and you'll stay free of blasters.
I'm no long distance walker, so I can't tell if that works. But I tried urine as an enema and it is really nice. It cleans without irritating the colon. It is soothing instead.
I wouldn't keep it for long before use. I collect my morning urine in an isolated can to keep its temperature. Then I empty my bowel, the normal way first and then with an enema of warm plain water. Then I pour the urine in the enema equipment, have it in my bowel, and keep it for an hour.
I don't trust anything that irritates, but I trust this enema. It just feels good.