definite purpose contactors are nothing more than heavy duty relays. They are made specifically for switching large loads on and off.
I use one inside my control panel as a manual "OFF" switch for my 5500W element.
It is controlled by a 120V signal. I have a small switch on the front of the panel that passes 120V to the contactor's coil to activate it.
edit:
You wouldn't want to do a lot of switching with the thing, I wouldn't think. I don't think they are meant for high-usage. I flip mine on at the start of the brew session and then the PID controls the on/off of the heater element via SSR. When I want to absolutely turn the element off, I flip the contactor off.
That's the most common usage for the contactors. They aren't DPDT though, they're DPST (two poles, either on or off).
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