Cryo treatment of steels changes the crystalline structures of the steel. Most importantly converting austenite to martensite. Martensite is a harder more finely refined structure, and should be harder and longer wearing. I know there are also claims that it stress relieves at the same time, which may help fight wandering zeros.
I've never really looked into its use on barrels, but I use it in knife making with some stainless steels where it has a pretty profound effect. Raising the Rockwell hardness a full point or more from the as quenched hardness is normal.
That being said blade steels are made with much higher carbon contents and are probably more effected than barrel steel would be. The stress relief may be really beneficial, but I don't know if the lower carbon steel would show enough of an increase in barrel life to offset the cost of the treatment itself. Hopefully someone on here will have some first hand info on its effects.