Gorse got most of it. The magnums, and some standard cartridges use the slower burning powders too, and so need the longer barrels.
One of the things I like about a 308Win. is you can use a little shorter barrel without losing much velocity. I read an article by Finn Aagaard some time ago in which he cut
the barrel of one of his 308s down. He lost approx. 30fps cutting it to 20", but when taking it down to 18-1/2" the avg. loss was about 75fps.
Personally, I'd like to see 21" barrels on standard cartridges, and 23" on magnums.
Carbines at 19.5" (18.5 is a little too short).