why do we use wads in bp revolvers?

Here's another vote against ever having had a chainfire caused by lack of a wad behind or grease in front of the ball. As long as the ball is large enough that there is a decent ring of lead shaved off on loading there's just no way for flame to get through unless the chamber flares behind the mouth leaving a gap around the ball. Improperly seated caps or caps of the wrong size are another matter.

John

 

[arcticap]

I punch my own wads/cards from juice cartons so I have no complaint about their price.
At the very least they help to seal the powder from melted lube and moisture.
I don't know if they help to provide more consistent velocity or not, but they might.
Since I load using a press, the stiff cards help to compress the powder very uniformly into the chambers.
And some powders like Pyrodex and American Pioneer do perform better with extra compression.
Since some antique & modern guns have corroded & pitted chambers, and some balls can have defects from being poorly cast using impure lead or a bad mold, a tight fitting barrier is like "free" insurance.