depends on the car. They all do the same and bring the coolant and eventually the block and oil upto about 80 Degrees. Some heat interior for you too.

in wear terms, if you do a lot of short or town journeys then it probably makes sense as your engine is mostly warmed up straight away, and the 10p a cycle to heat up probably saves more than that in fuel that would be wasted in the inefficient cold run/warm up period.