It may depend on what powders you're using, some are more prone to overcharge than others.
If you watch the Chargemaster bumping the drop tube when it's trying to dump that last tenth of a grain, you can see it happening and WHY it's happening. The reduction insert puts a stop to that action.
Some guys say that it helps make the Chargemaster more precise. I can see why some may believe that but I'm not 100% convinced...but the fact that this will put a stop to those 1/10- and 2/10-grain overcharges makes my own reloading life a little better.
Well done in the DIY fix with the tools you have available.
I actually PMed the original designer on Sniper's Hide and copied his dementions with a piece of brass solid stock, in the lathe. Before the insert the Charge Master would overthrow half of the charges, with Varget, H-4350, and Retumbo (all cylindrical powders) after the insert it over throws 5%.