Another more practical reason (besides free bags for use around the house) is that produce is often wet from the misters and refrigerated items condensate once you go outside (especially in the south). Double bagging helps prevent the bags from tearing if/when they get wet. Also, for people buying lots of canned goods, single bags can rip if they’re overloaded. Cashiers and baggers will still double bag plastic bags when they are filling it with a lot of heavy items.

Another reuse for brown paper grocery bags was DIY textbook covers.