I'm sure other can give you a lot more detail.
Apart from the obvious cost advantage they also serve for testing barrels as you know the scope, bedding/glue in is OK.
On push feed actions you can also be set up to switch barrels between actions.
This is very simple to have done. Firstly an action (say Rem 700) is faced off. Then a barrel is fitted. That barrel is then put on other actions and those actions are faced off so that barrel is headsapced and fitted.
This is done for each action.
Up till about 5 years ago I had several bench rifles on larger calibers like 270 and 300 Win, all Rem 700 actions with Jewell triggers.
A mate of mine has several Rem 700s and his actions were faced off to match mine. He experiemnts with a lot of different calibers. When he got a new barrel we would put it on one of my actions to test and load for. Much easier with the wide benchrest forend and 2 ounce trigger. When finished, we would then put the barrel on his rifle.
If the rifles are glue ins, you can also use a washer as a spacer between the barrel and action. In fact I used to have several of them that varied by about .001" in thickness.
Mike
[This message has been edited by Mike375 (edited 07-18-2001).]