If you live in a place with cold winters, it's pretty handy to be able to start your car as you leave the office. By the time you've walked to your car, it's nice and warm and the engine is running optimally.[1]
1. I know modern engines claim not to need to be warmed up, but I think that's a load of crap. Oil and other lubricants at very cold temperatures need to warm up and there's no evidence that it hurts anything. And it only uses a little sip of gas.
If you think oil and lubricants need to be warmed to a certain point to not harm the engine, why would it be any different if you were idling cold or putting it under light load cold? It wouldn't really. Old cars needed to be warm for the carburetor to provide the correct air and fuel mixture. It didn't have anything to do with lubricants. Once cars became fuel injected it became a waste of gas to warm up the car idling. The engine actually warms up faster being driven.
A block heater won't get the interior warmer for you to get in and block heaters require plugging in which won't be an option at the office parking lot for most people.
1. I know modern engines claim not to need to be warmed up, but I think that's a load of crap. Oil and other lubricants at very cold temperatures need to warm up and there's no evidence that it hurts anything. And it only uses a little sip of gas.