Directional tires are usually for traction. This is controlled by tread pattern that gives them the ability to self clean. Some are directional by design at the job at hand. Off road, all terrain. highway mileage. winter / summer etc. Depending on the design is the way the tire cords are layered. Also the rubber compound. If it has a arrow on it make sure it is in the right direction what ever you have to do. If you want to see cupped duel rear tire visit your local truck stop or check out the 18 wheeler next to you in a rest area.