I don't really "get" one dog having 50 different collars...but I also don't "get" why some people have 2000 dvds on the shelf, or a binder full of baseball cards, or 3 computers, or an extra car. I'm sure plenty of people don't "get" why I have 7 pairs of boots, or why I spend a lot of time fostering dogs, or why I have photographs and books everywhere.
It's just a hobby, something people like to collect, for a number of different reasons.
That said, I like collars, and I want my dogs to have nice collars, and I like the historical and artisan aspect of leatherwork. Also, I like the collars to sort of reflect the dog's personality a bit...especially working with bully breeds. If my pit bull has a collar with rocketships or flowers or peace signs on it, it looks way more approachable and friendly than a dog with a spiked collar or even a plain leather or nylon collar...and if I'm taking my dog to be an ambassador at some kind of community event, say, or to watch my friend's kid play soccer, I want to come across as non-threatening and friendly.
The same way putting a "tougher" collar on my dog to walk around a sketchy area at night might prevent people from approaching us.