Useless as it probably is, there could have been a lot of thought put into it. Some thinking along the lines of it giving primer 2x exposure area to the charge while reducing the powder and/or pressure loss through the vent. I.e., simply drilling out the vent 50% larger does the same thing, but they may have hoped that the flow of gasses back through two small holes would be more restricted than the one larger hole. Many similarly useless things are done these days, e.g., ported barrels (my opinion only). 1000's of ideas will be tried and found pointless for every one that becomes historically correct standard practice.