@Iluviaporos - It actually made urine into a bankable commodity for a while, because the dyers were willing to pay for it.
It's the same as when guano was one of the best sources of fertilizer and people went crazy for that and paid massive amounts for it. I believe it has uric acid rather than urea though.
What I've always wanted to know is how the people originally came up with this solution. It doesn't seem like a very intuitive thing to me, to soak your clothes in urine!