In areas with "real" hills it often can be advantageous if not outright necessary to force engine braking while descending. While the need may be slowly on the wane, what with automakers adding yet more smart programming, it still applies to a good deal of vehicles that are still on the road, years after manufacturer--and not getting updates to their original transmission programming.Why - unless there is a situation that warrants it let the transmission do what it's programmed to do with respect to upshifts and down shifts.
Plus, some people like to think ahead and would like a bit more input to the car operation than others. If I step on the gas, get into an intersection, let off and make a turn, then need to get on it... there's no need for the transmission to upshift, only to have to downshift. This happened a lot on my commute, in one spot, and it kinda bugged me. The transmission does not have eyes, much less can read my mind. It does not know that, if I were driving a manual transmission, that I'd never upshift in this spot, as I will need to be on it in a moment.
And some cars are really awful with their lag, making up their minds, figuring out what gear to be in. Some are faster, some are smarter, and some should be in the junkyard.