An engine block heater helps keep your engines coolant warm through the winter so it keeps your engine block warmer so it won't have to sit and warm up too long.
The engine block heater also helps your heater warm up faster so you can deice and defrost your windshield faster.
Your vehicles heater uses the engines hot coolant to provide heat inside the vehicle and to defrost your windshield so the faster the coolant heats up the faster your heater will work.
A block heater can also use a lot of electricity and when I had a block heater plugged in at home my electric bill went up an additional $60.00 per month.
So I found it was not worth it and it was cheaper to just let the engine idle a little longer to heat up in the morning.
Engine block heaters are nice and they do work but they are expensive to operate as they usually use around 1500 watts of electricity per hour.