Freshly roasted coffee beans give off a great deal of carbon dioxide, for several hours after roasting. To prevent this "off-gassing" from bursting their bags, coffee roasters used to hold coffee for up to 24 hours before packaging. This meant that your coffee was already a day old before it even left the supplier.
By using bags with one-way air valves, coffee can be packed immediately without off-gassing worries. A bag with valves tells you that your supplier is interested in providing a good product, and that your coffee is as fresh as possible.