I'm not a BP shooter, but have done some reading on it, and found some very good shooters use over primer wads. But also found some good shooters who don't too. 
Here's a read I found long ago and saved, which explains why. Whether you do it or not is an individual choice:
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Reduces the primer power so less chance of the charge moving early from just the primer going off.

I believe that the idea is that everything could start early(by milliseconds) including the projectile, and were only talking a thou. or two.

Some smokeless powders do have similar burn rates to BP, so can end up with the same unburned powder, or dirty bores from the same thing happening. In BP, I understand the issue of the bullet moving out the bore when a hot primer is exposed to the powder, can create a dirtier bore as the bullet moves down the barrel, and at times exits before complete ignition takes place.
I've had some smokeless powders I thought might benefit from shielding the primer, and burn cleaner and more complete. I usually just changed to a faster powder, rather than complicate my loading procedure.

At one time both black powder primers and smokeless primers were available. Smokeless requires a higher brisance than black. The overprimer wad is intended to reduce the brisance of current primers back towards the level of the old black powder primers. Some users put the wad (actually usually a thin piece of paper like tracing paper) between the primer and the flash hole and others put the paper inside the case over the flash hole (which also keeps the powder out of the flash hole and may promote more even ignition.)

The ODG's where pretty smart, as soon a smokeless came along, they added some to the old, black, dirty stuff that they had been using. 

There didn't seem to be any "rules" about shooting those steel Buffalo's at that time and I'm pretty sure that they would have used duplex in long range matches too, if there had been any at that time and I know they would have used the 1878 Borchardt. 

Anyone that used it after the start of the 20th century, used it because it was cheap. It's NOT cheap any more, it cost much more in both, time and money to shoot it now.

That said, I like BP but, only with duplex loads (still much harder to get accuracy) but, the matches go smoother and faster.

I've offen wondered where the Buff Hunters got there news paper for their primers and did they use a hammer for their compression dies?

Sorry, kinda OT, I know ( )

Frank

The old Ideal #6 duplex powder measure came out shortly after smokeless powders hit the market. I'm sure the shooters back then needed a duplex measure, or Ideal wouldn't have made them.