BladderScan

I saw one of these at work yesterday. The device's name caught my eye. Pardon my ignorance on the matter, but what the heck is a BladderScan for? What possible use necessitates an entire machine? I mean, this is a very specific niche. The website tells me that it estimates bladder volume. Knowing this fact just leaves me with more questions. Why can't a conventional ultrasound machine be used for measuring bladders? What are they hoping to find? Why is it helpful to know how much urine a bladder contains? What the heck? Perhaps I underestimate the importance of bladder health. Maybe someone with more knowledge of urology can help me out.