On manual focus cameras with one-touch zooms, you get the focus easier at the longer zoom, than just slide it at the desired focal length. But be careful, there are very few zooms that do not preserve the focus when changing the focal. Between these, if I recall, is a short zoom from Vivitar S1 (Kiron) 28-80mm (or something like that). Although those few zooms are optically beautiful. Or am I wrong, and this is a two-touch zoom? I’m not sure anymore, as I avoided it years ago for this same reason.
As for AF cameras, using a one-touch zoom means to deactivate the AF. Two-touch zooms are best used with AF, as one-touch zooms are with MF.