The animal would not be constantly dropping trackable scent that would lead predators to follow him. If the animal is a predator, it would constantly be dropping scent that would warn prey that it is nearby.

Also they need something to intentionally mark their territory.

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I mean, you'd have a constant stream of urine leaking out, making everywhere you go stink of urine, including the places you really want to keep clean, like your nest. Also, it would get on you all the time, making you reek of urine as well.

Also it is useful as a water reservoir. For instance, reptiles that live in the desert tend to have large bladders.

Because some of that water can be reabsorbed in an emergency. Yes the water is heavy and has to be heated to 37 C but it's not wasted. If you constantly leaked water you would have no where to pull extra water from when you become dehydrated due to exercesie/losing blood/getting lost. Evolutionarily it was probably advantages to have more water storage rather than less .