They were used to lay down a field of cover fire you didn't want to be on the receiving end of....lol

I don't really know but back in the early 90's when we were turning in our revolvers for hi-capacity auto's there was something in law enforcement about having extended capacity magazines. I traded my 4" model 64 for a 659 and a required piece of equipment was a S&W 20 round magazine. This was of course until Ramline started making 18 round flush fit magazines and then we all had to have them. It would drive the firearms instructors nuts when they laid out the duty ammo, leaving 43 rounds in a 50 round box (since we started with 14 round magazines when 659's were first introduced) and we would have to have them draw more duty ammo when we showed up asking for 12 more rounds.

I think I remember somewhere that the 15 round 1076 mags were for call out bags or the bail out bags of the time, an extra piece of insurance if you though you may need it. I am sure that LadyFed can answer it better than I.