This is one of those things we do because our mom did, and we have no idea if it's actually doing anything. Our guess is no, but we still do it — and we found funny little product devoted to the practice. See it below...
This is from The Container Store, and it is supposed to let air reach your open baking soda while keeping the baking soda "fresh." We're not exactly sure how a ventilated, plastic lid keeps the baking soda any fresher, but hey, if you're interested:
We've read a bit about how baking soda can absorb odors (and covered it in the past), due to its alkaline nature that can neutralize acidic odors (again, we're not scientists). Most of what we read, though, notes that it doesn't absorb odors very well — not well enough to really "freshen" a fridge. Also, the powder tends to form a crust or film that hinders its effectiveness.
Maybe shaking some out on a shallow plate (and changing it every so often) would work better?
Anyone have any other tips? Do you think this common practice really works?