Getting your clubs set up correctly is vitally important to ensure they perform to their best.  You can have the best swing in the world, but if your clubs are too flat or too upright, too long or too short,  you are going to have to make compensations in order to hit the ball correctly.  If you consistently miss green on one side or the other then there is a good chance that your irons might be set up either to flat or too upright. If you are struggling for distance or hit the ball too low or too high, it may well be that your equipment is set up badly for you, either in terms of weight, length, swingweight or clubface loft.  For the importance of getting properly fitted golf clubs have a look at these video's by leading clubmaker, Tom Wishon

If you miss to the left, especially with the short irons,  your irons may be too upright.  The toe of the club will be up in the air, causing the face angle to be aiming way left and sometimes will cause the ball to fly to far.  If you miss shots to the right, it may be that your irons are too flat, causing the face angle to be aimed right of where you think it is. 

Another really destructive aspect of having improperly fitted clubs will be the tendency for the golfer to adjust their setup to accomodate for this fact.  The clubmaker provides much of the information needed for us to set up to the ball correctly.  The distance from the ball and the amount of spinal tilt is dictated by the club being used.  If your clubs don't fit, your setup, posture and address position will be wrong.  You will then have to make adjustments during the swing to accomodate.