Although surfboards can handle a lot of stress and punishment when being put through the ringer, they are fragile out of the water. It doesn’t take much to crack or ding that board, and can happen in the most unlikely ways… a water bottle rolling around in your car, your dog playing tug of war with your leash, etc. A decent surfboard bag is a protective layer from dings, knocks, scratches, sunlight–absolutely everything when it’s out of the water.
Keep your board in its bag at all times. That way, you can easily toss it in the back of your truck or car without worrying, you can chill on the beach for a few hours after a session without worrying about the sun (on your board, that is), and you can safely keep it in your house or garage knowing that if something falls on it or it falls over, it’ll be safe. It also helps to keeps it cool in hot places. Too much heat can lead to delamination or bubbles, where you find the “skin” peeling from the foam in certain areas. Plus, you can toss wetsuits, board shorts, fins, etc. inside, making the bag do all the work for you.