If I remember correctly, there are 7 states that Ford (and most other automakers) consider "northern tier". Truck/cars that are slated to ship to those states come from the factory with a block heater installed.
The factory installed block heater takes the place of one of the freeze plugs in the engine block. (Freeze plugs are designed to "pop" out, should the engine coolant ever freeze/expand, and prevent the engine block from being cracked/broken)
There are several varieties of engine heaters..... a "block heater" is generally the type that replaces a freeze plug. There are also magnetic versions that are intended to "stick" to steel oil pans (not many of those being made any more), dipstick versions, and even versions that are installed in the radiator hose.
Unless you live in an area where sub zero temps are common, you won't gain anything by installing one. When they really shine are on those -30+ nights.