The device is set up like a duplex receptacle, but has a 15A single-pole switch in one half, and a single 15A, 125V receptacle in the other half. They can work in conjunction with one another, or they can be connected and used independent of each other.
Here is an actual picture of a Leviton Switch/Receptacle Combination Device.
I will discuss three different possible scenarios where a device such as this would come in handy.
In your garage or shop, you would like to install a task light, like a florescent light fixtureover workbench. However, the walls are all finished so getting a new wire to a switch and then up to the light is not that simple. However, you have a receptacle just above thebench that you can get your power source from.
You can change this duplex receptacle to a switch/receptacle combination so that you still have power in the outlet, but can use the switch to control your new overhead task light. Fishing in a wire from the receptacle to the light fixture is fairly easy, so this is how you would wire the device in this situation.
A typical example of this situation is if you had the same scenario as above, but with a 3-wire circuit, such as in a kitchen split receptacle, and wanting to add some under-counter lighting for example.
Just as the receptacle that you replaced will have the tab on the hot side removed, here we have to remove the tab on the device as well. This will put the light on a different circuit as the receptacle, but again will give you the desired result of a usable receptacle, and a switched light fixture without a difficult wiring renovation.
Keep in mind that this isn’t the best situation, as you now lose the required amount of circuits in the kitchen. However it is an example of how we can have the switch and receptacle operating independently from one another on different circuits.
Here is a situation where your desired result is a receptacle controlled by the switch in the combination device.
Let’s say you have a task light for your shop, and it is cord connected and designed to be mounted over your work area and then just plugged in to an outlet. Instead of plugging the light in and out every time you want to control the light, this will allow you to keep it plugged in and use the switch for control.
So there you have three examples of where a device like this can come in handy.