There's actually a bit more to it than just "all the wheels pointing straight ahead". The front wheels in particular usually have three settings - camber, caster, and toe. Camber is the vertical angle the wheel meets the road at ( left - right tilt ), caster is the steering angle ( forward / aft tilt ), and toe is the horizontal alignment of the wheel ( toe-in or toe-out ). If any or all of those adjustments are incorrect ( due to normal wear and tear, or hitting something on the road ), your tyres will wear in an uneven and premature way. These settings need to be periodically adjusted as suspension and steering parts wear and settle over time.