Why do people dock dogs tails?

Some advocates claim there are some benefits to working dogs like cattle dogs and Aussies, or some hunting dogs. The idea is that the tail is prone to damage due to their line of work. However, I've seen studies done showing that the incidence of tail injury in undocked dogs are lower than the incidence of surgical complications for docked dogs, meaning that more problems are caused by docking than are prevented.

Today, tail docking is more cosmetic than anything. Breeds like dobermans, German shorthairs, and other dogs where the breed standard has docked tails, it's just to get the classic look. It can be used for practical use still, like hunting and herding, but that's not very common. I have an English pointer that has broken his tail so many times, that getting it docked might have been better for him, but on the other hand, my German shorthair had a botched docking and now the fur won't grow over where it was docked. It's completely owner preference at this point.

I agree that it is usually aesthetic or tradition. I had two Border Collies with tails that did some shepherding. I then got a Mini Aussie from a local breeder but I wanted to keep his tail since my BCs ad theirs. When the Mini Aussie got older his tail started curling over his back. By then I did not have sheep for him to herd anymore but I could tell that if his tail got caught in some sticker bushes it could get broken. The BCs tail is used as a tool in herding and stays fairly inline with their bodies thus not very prone to breaking.
I have also heard of BCs tails getting docked in very rural areas to distinguish between wild canines and domestic ones.

I know that for pit bulls, they have powerful tails. None of my dogs have had docked tails, but my furniture has suffered from it. Dented refrigerator, paint chipped off the walls, and my first dog even broke a mirror with her tail; only the furniture has been damaged, I've never experienced any broken or split tails. Some may do it for looks, but it's practical for more than just health reasons. Especially if you live in an apartment and you want to get your security deposit back.

Some do it for aesthetics, others do it for working dogs or to protect injury prone dogs. My friend has Great Danes that wag so excitedly that they have broken their tails multiple times on door frames, the fireplace, furniture, etc. She decided to have her new puppy's tail docked to save him from future problems.

There are claims that docking benefits a dogs skills like hunting etc. But now it is just a breed standard for some. If you get a dog that the standard look is with a docked tail then you will look out of place.

A lot of it has to do with aesthetics.