This will help acceleration in a rear-driven car. That said, the vast majority of cars out there are front driven, and weight makes cars handle worse, and brake worse. So yes, your truck or old Buick Roadmaster will get going better in the snow with some sandbags in the back, but it will also slide around significantly more once you're on the road. Bad idea.
There's a reason that WRC cars don't load up on sand bags before races.