It may have more bearing on the vacuum controlled cruise? Since my swap eliminated that, I hadn't reinstalled the ball. The only things vacuum hooked up are the vent selector and the brake booster. Then, during some hare-brained trouble shooting, I installed the ball, but mine is a single port. I teed it inline to the vent controls, and noticed no difference. Your results may vary.
The idea is to be just as stated. A reservoir for vacuum. Similar to the way the brake booster works. As some of the vacuum-operated items are doing their thing, the hope is that the reservoir keeps the vacuum signal steady in relation to the engine vacuum. If vacuum fluctuates, you may start having those vacuum operations go snake shiit on you. Thus, customer complaints. Thus, engineers get yelled at. As far as how effective they are.....I guess the engineers figured they needed it.