A 3rd party dock:
1. Is small.
2. Is cheap.
So, the first reason, it is small:
Imagine you want to carry the dock in your pocket when visiting a friend or something, well, unless you have huge pockets it is difficult to do that with the original dock. You could still use the electronics of the original dock, and put that into a smaller case, but take into account that that would cost you some $80+ (for an original dock) plus the cost of the smaller replacement dock.
For the second reason, it is cheap:
Suppose you have multiple TVs in your home, and you move your Switch to some other TV often, perhaps from between your bedroom and your living room, or whatever. You either have to disconnect and reconnect the dock all the time like an idiot, or you can buy a second dock. And a Nintendo dock is $80+, while a 3rd party dock may cost $20-.