Speed and price (but not speed on the actual barrel swap). A smith doesn't need your action, so your action doesn't need shipped both ways. Since a particular action isn't needed, some are pre-cutting popular options and have them on the shelf. You can have a chambered barrel shipped to you today just like a box of bullets. Additionally, most smiths I see charge less for machining a barrel for a nut setup vs a shoudered install.
NSS has piles of popular chamberings on the shelf ready to ship, you get a chambered Creiterion or Shilen for about the price of a high end blank. Greg Young will chamber any of his top shelf in stock barrels for you in your choice of chamber for $180, and do it in 30 days. Just a couple examples, I think the trend is going to continue and is a great option to have for high volume shooters. I'll be screwing my first one on tomorrow...
