A golf club has only three components, a grip, a shaft, and a head. Most golfers consider the shaft to be the "engine" of the golf club, and thus the most important component.
Contrary to popular belief, the shaft is neither an engine, nor the most important part of the golf club. The head - by far - is the most important component.
The shaft doesn't snap the ball forward (like a slingshot), or square the club face to the target at impact (as is it knew where the target was). And, needless to say, a very expensive shaft does not do both.
Also, a stiffer shaft doesn't stabilize the club head at impact, as so many golfers believe. Remember, the club head is in contact with the ball for only about 5/10,000th of a second. On off-center strikes, the club head will twist as though it was not even connected to a shaft. By the time the shaft stabilizes the head, the ball will have already launched off the club face.
Having said all this, the shaft's length, weight, flex, and bend profile will certainly affect the feel and playability of the golf club. Therefore, selecting the appropriate shaft specifications is an important step in the club fitting process.
Don't be misled, however, into believing a shaft can do more than it is capable of doing. The shafts overall importance is greatly overrated. It's not an engine and it's certainly not as important as the club head.