EXPLANATION
It's not essential, but is almost always used when taking-off in a commercial aircraft. The reason for deploying flaps is very simple: to create lift. This is part of the the reason why aircraft take-off into the wind. ​
Lift is created when air passes over the wings of the aircraft, which is made easier when it's blocked by flaps underneath the wings. The flaps simply create lift for the aircraft, allowing it to take-off sooner. Generally, at most airports with fair weather, a pilot will set flaps to between 5 and 10 degrees.
However, this may vary in certain circumstances. As seen in the image above, by using less flaps, the aircraft will travel along the runway further - this technique is often used when an aircraft needs to climb higher, faster. This may be because there is an obstacle or hill shortly after the runway, by having less flaps, it reduces drag and allows the pilot to raise the nose even higher.
WHY DO AIRCRAFT TAKE-OFF INTO THE WIND?
The formulas are quite complex, so here's a basic example to help you: If an aircraft needs 150 mph of airspeed to take-off, without wind, the plane has to accelerate to a ground speed of 180 mph to lift off, but when you have a 30 mph headwind, the plane only has to accelerate to 150 mph, thanks to the extra boost it gets from the headwind.
USEFUL VIDEO
Learn how to take-off a Boeing 737-800.
Including deploying flaps for take-off.
REMEMBER
Generally, the shorter the runway, the more flaps you need. But don't over do it, no more than 15.