It Helps Protect Against Dust and Debris
Cold weather is not the only reason people cover their mirrors. Cars that sit outside collect all kinds of grime: road dust, grit, pollen, salt spray, leaves, and tiny bits of debris blown around by wind. Your side mirrors take more of that than you might think because they stick out from the body of the car.
A bag gives the mirror a cheap temporary layer of protection while the car is parked. It can help keep the glass cleaner, especially if you are leaving the vehicle near a busy road, a dusty campsite, a construction area, or a gravel lot. When you return, you are less likely to find the mirror filmed over with dirt or scratched by particles rubbing across the surface.
This matters more on solo trips because you rely heavily on your own visibility. Clean mirrors help you make better decisions. A bag will not protect the whole car, of course, but it can save one of the small parts of daily or frequent driving.