As Wiirachay explained, it's about minimising groundspeed rather than airspeed. Airspeed is defined as the speed of the air moving across the aircraft's wings, whereas ground speed is determined by the vector of the direction that air travels in addition to the vector of the aircraft's own movement relative to the air. The airspeed you need for takeoff is more or less fixed depending on the aircraft and its flap configuration, but the groundspeed corresponding to that airspeed can vary greatly depending on whether you have a headwind, tailwind, crosswind or no wind at all.

Aircraft always try to take off in a direction that gives them the most headwind possible - it takes less effort (and therefore less power, less fuel and less runway length) to reach the necessary airspeed for lift-off.