It is clearly there to keep the primers separated and I believe it must be to help prevent flash over
The original packaging is actually designed to stop a propagation when 1 of the primers goes off. Once you take them out of the original packaging, all bets are off.
Here is the actual propagation suppression that worked as designed:
Only 50 some of the 1000 went off. In your glass jar, you wouldn't have been so lucky.