This is an old school trick people used to use, in the end though IIRC while it did sort of work a little bit, it wore off fairly quickly and changed the tuning (lowered velocity i believe), so it wasn't really a thing to do.
I agree with the above posters. When I was starting out in the mid 90's I tried it. The reason to do it was because the paint sucked so bad back then it was only a matter of time before you had a barrel break. The theory was when you break a ball in the barrel the paint would bead up and be pushed out of the barrel with the next ball, thus making the barrel self cleaning. While it does work kinda, it never really self cleaning, also its hard to do properly and does not last all that long so you have to reapply maybe 3 times over the course of a case of paint. So in conclusion your much better off just running dry