Well from an engineers perspective (I'm sure many ppl on this forum will agree/disagree with me), when the car is cold, nothing is up to operating temperature, which means that everything is close to each other (contraction and expansion due to cold/hot). So that's one reason why you should never rev the engine really high when the car isn't at operating temp. Also, the materials in the engine will have very different mechanical properties at 5 degrees than at 60+ degrees centigrade... Elasticity, yield point all changes. Everything is designed with a factor of safety, usually around 2; but sometimes, the conditions are just right for a failure to happen or seriously damage the engine. No one wants to scratch the cylinders because they weren't patient enough for the car to warm up
And the car usually is broken in to eaven out any tiny (micrometers) imperfections in cylinders and other areas around the engine. This is done to ensure a perfect fit of piston with cylinder. Imagine the following scenario:
You have a new car and you don't want to break it in. By some luck, it happens that your cylinders were just a bit too small and the piston rings are rubbing furiously against the walls. If you don't take it gentle, it'll scratch the cylinder walls and damage both the walls and piston rings. Piston rings are replaceable, cylinder walls, not so much. The same goes for all moving parts, bearings, pumps, compressors, etc.
It's like wearing new leather shoes, it's a tight fit at first and then it just loosens up and fits perfectly. Hope that helps.
I'm the kind to always break in my car, I don't want to create unnecessary damages because I wasn't patient.
Just a few examples, I'm sure many ppl can come up with more or argue against what I said.
I hope I got most of that right!