Urine is one way in which the body gets rid of waste. The urinary system was developed when all animals lived in the ocean. Access to water was not a problem, so the system was profligate with it. Need to add some water for dilution? Go for it! Plenty more where that came from!
After animals left the ocean, water became a problem. It was desirable not to dilute the urine so much. Was the solution to adjust the amount of urine added to the water? It was not.
Instead, excess water is added, but a later change takes it back out.
The urinary system seems kind of kludgy.