I just looked it up in Martha Christy's book: YOUR OWN PERFECT MEDICINE.
I am paraphrasing, but it says that often times people with Urinary, Bladder or Kidney infections get them because they do not have enough concentrated Urea in their urine. And Urea is the substance that has the anti-bacterial properties.
So people who drink Urine when they have a tract infection are forcing their bodies to have more Urea and fight the infection much quicker.