When you remove the little plug (#3 in the diagram below)...it allows you to lubricate the mechanical workings of the door handle that may get frozen by water that runs down the interior of the door when temps drop way below freezing.
Using a lubricant that helps dispel water...helps prevent the freezing and broken door handles that occur during this time of the season.
I use a product called
TRI-FLOW that has TEFLON in it...which helps remove moisture/water...which helps with the freezing. I also try not to wash the vehicle if I don't think that any residual water/mositure won't evaporate before temps dip below freezing. And if you have rain/freezing-rain, sleet, snow, etc that may melt and run down the door along with impending freezing rain...this is the reason for lubricating the door handles and removing that little plug and using a product that can help dispel/displace water/moisture from this area.
These are my annual winterizing products...the TRI FLOW is used on all parts that move or can bind (locks/hinges/etc) and the silicone is used on all gaskets and rubber to help them not freeze together, as well: