Why do we need to warm up the engine before using the car on a chilly morning?

Actually you don't. At one time it was important when we were using 30 weight oil but now a multi-viscosity oil solves that problem - unless where you leave it gets to 30 below. Warming up the engine is for those that need heat for comfort. You are better off starting to drive and not do any jack-rabbit starts for a mile or so. That will use less gas and the engine will warm up faster as well as pollute less.