The following is a quote from J. Marshall Stanton of Beartooth Bullets .
In most guns, there will be at least a modest increase in accuracy potential, to often times dramatic improvement in accuracy from increasing the uniformity of the bore dimensionally, and eliminating the constrictions which are common under dovetails in barrels, and barrel bands.
Especially in revolvers where the threaded barrel shank is screwed into the frame of the revolver, dramatic improvements in performance may be achieved. This is particularly true where cast bullets are used in the revolver, because the constriction under the threaded barrel shank (which can be from .002"-. 004" depending on make and caliber), acts like an undersize, bullet sizing die, thus sizing the bullet down under barrel groove dimension, creating an improper bullet to barrel fit. Not only does this condition deteriorate accuracy; it also promotes barrel leading. Fire lapping, properly done eliminates this condition.
I couldn't find an easy quote from Vernal Smith of LPT, a sponser here on GBO, but I know he has written something very similar, and I doubt you'll find a gunsmith who would disagree. But lucky for you, here we have experts who can tell you that is all "bunk".