Whether a car needs to be 'warmed up" before driving in the winter is a very debatable topic so im not going to ask that.
But a related question is... why does a car's idle rev drop as the engine gets up to temperature?
The idle is held high while cold because it simply won't run at the regular idle speed when cold. It's not to warm it up faster, it's just to keep it running. Cold oil is thicker and harder to pump, and cold engine surfaces tend to condense the fuel into larger droplets which resist ignition.
To compensate when cold, mixture is very very rich, i.e. MUCH more fuel than normal for the amount of air taken in. That's what a choke does on a carb, block intake air (literally "choke" the engine) without changing the fuel delivered. Electronic fuel injection compensates simply by injecting more fuel.
If you remember carburated cars, you know how much easier it is to start a modern fuel-injected car when stone cold than it used to be with carbs. To cold-start a carburated car, first thing you did before turning the key on was to floor the accelerator and let it back up. This closed the choke and allowed a temperature sensitive piece in the throttle linkage to catch, setting the high idle, and you then started the car with as little gas pedal as you could get away with. As the car warmed up, that temperature-sensitive piece in the linkage moved aside, and after that happened letting off the gas let the idle drop to normal. If you just sat in the driveay too long, the engine would run faster and faster until you tapped the gas pedal to release that linkage piece.
With electronic fuel injection, you don't touch the accelerator, you just turn the key and start. The brain knows the motor is cold, triggers a valve to let more air in past the throttle so it idles fast, and it sends more fuel through the injectors to make it rich. Same effect as a choke, without actually having one.
With either system there's an additional result of a large amount of unconsumed fuel going into the exhaust system, known as unburned hydrocarbons, which on modern emission-controlled engines must be taken care of somehow.
Anyway, it's not so much that the idle speed is dropped as it warms up, it's that it's held high while it's cold.
As for letting it warm up before you drive, you don't need to. On the older cars, they would sometimes run very rough when cold. Sudden throttle would cause it to stumble, as the carb could not figure out the right amount of fuel to mix. Those cars wanted to warm up a bit before they moved. With modern FI you just back out of the driveway and go about your business. The ECU makes everything work correctly, and the best way to warm up a modern engine is to drive it. No full throttle or high revs while cold, though, for best engine life. (Remember that hard-to-pump cold oil!)
