They're a totally valid solution for people who want access to the brakes from the tops. The bike can fit you fine; you can prefer to ride on the tops for at least a portion of the time,and you can want access to the brakes from there.

They're used quite a bit in cyclocross, by professionals with perfectly fitting bikes.

Most people find they don't need them because they can get to the brakes quick enough anyway,and don't tend to ride the tops where they're likely to need to brake.

But if you want that security, it's a valid option, and can make people coming to drop bar bikes for the first time more secure.