It just hurts the engine.

In the plunging temperatures of this icy Irish winter, it's common practice to leave the car on for a few minutes before sitting in and driving off, not only to warm the interior but also the engine in case it got frosty. Many have said that this warming up ritual prolongs the life of a car engine but it seems doing this causes more bad than good. According to Cosmopolitan, warming up the engine actually gets rid of oil from your car's pistons and cylinders in the engine. Your car's combustion engine uses pistons to mix air and fuel within a cylinder, the mixture of which is ignited to start the engine.  With a cold engine, gasoline is more unlikely to make the correct amount of air for combustion, as most engines have sensors that compensate for the cold anyway. Engineer for Argonne National Laboratory, Stephen Ciatti confirmed the news to Business Insider.

"That's a problem because you're actually putting extra fuel into the combustion chamber to make it burn and some of it can get onto the cylinder walls." "Gasoline is an outstanding solvent and it can actually wash oil off the walls if you run it in those cold idle conditions for an extended period of time."

It's recommended that instead of leaving your car with the ignition on, to drive slowly for 5 to 15 minutes as the engine warms up.