Why does storing bread in a plastic (grocery) bag keep it fresh longer?
Water, water-vapor or moisture can't penetrate the plastic bag. Nice tender bread contains a lot of moisture. That moisture, all of it, will evaporate into the relatively dry air in your kitchen if it can. Bread (especially a baguette) depleted of moisture becomes rock hard, as you know. If you cover your bread with a plastic bag, a little moisture still escapes into the air inside the bag, but it can't get out of the bag, and after that you get an equilibrium between soft bread and slightly moist-ish air, and your bread stays pretty nice for a little longer. 

The down-side is if you keep it too long, you may get mold taking advantage of the moist conditions in the plastic bag, and you will get moldy bread. Mold doesn't tend to grow in a dry environment (rock-hard bread).