It doesn't exist because it has an advantage. It exists simply because it does. It's a remnant of a language that existed before modern Russian. Such things like numbers and cases change slowly if most speakers of the language are natives, because we learn them in our early childhood and by the time we are old enough to consider whether it's practical or not it's already too ingrained into our brain, so any change feels unnatural. Most languages has some peculiarity of this sort.