I don't think that anyone will ask this question. But there might be some lazy dads or pet owners who might be thinking why I need to clean that. Why not let the mattress soak all the glory ( the urine of your pet or kids, whatever that caused it). For them, you need to clean for your own sake, and for the sake of your bed. You might be able to cover the stain using a bed sheet but the odor will not let you sleep. On the other hand, the urine isn't liquid. It is an acid. So, it has bacteria's which might be harmful to your health. Plus if you let the crib mattress soak the pee then it will also harm its quality. So, it might not last as long as you would have thought. Therefore, any sensible person would keep things clean even the lazy ones.