There is really two purposes to this.
One, is to control lights I cannot see from the switch. If I turn them on with a hardwired switch I know they are on. At other times I want them to come on via a timer. If I replace the switch with a control module switch, there is not definite "on" and "off" nor is there a small neon lamp in the paddle to show power is applied. As a result, I am in one of those situations where I cannot easily determine if the circuit on or off since I cannot see the lights.
Secondly, many of the duplex outlets in my home are wired such that half of the outlet is always hot and the other is switched. Some of these are used seasonally. And, while I could just plug in a module, and use as needed, at times I also want the capability of turning them on and off manually, without having to go to some other AHP, or control box to do it. Thus, using a double plug, (one in the switched outlet, one in the always hot, I could wire a small half duplex box with the guts of a module such that that one outlet then becomes either manually switched as it always was, and, in addition, the switched outlet could be supplied via the module while the other outlet is always hot, as in the past. I know it could just cycle the switch on the lamp, to get the module to turn on, but its across a dark room and .....etc.