Grinders are as important as your espresso machine, infact sometimes it might be more important the machine itself. The grinder dictates the rate of flow of the shot, and getting a good grinder makes this variable controllable.
Calibrating the grinder is prepping up the grinder to make those necessary adjustments, as time goes by you need to do cleaning and calibrating it as it might loose settings that you placed before. Just like a car, sort of a tune up, and its ready to go, in this case grind any beans you have. Now you can grind your beans to whatever brewing method you like. This way you know that you are grinding correctly for the surface area to water exposure for the desired method, and thus will give you the correct brew ratio for extraction or close to it.
The picture in this article shows 3 different FINE grind settings. On the left is the coarses of the 3, while on the right most is the finest, a notch finer to this can choke your machine.
The end cup of any pull is always the determing factor. Everything done right has the right result, pulling a shot correctly will result to good or prefect shot. But the term perfect is subjective, it is taste, this is when you dial your grinder. Stepless grinders and macro adjustments grinders come into play to address the issue. 1 second makes a difference in reaching your beverage shot, 1 second short makes another one. Its not the shot time but the rate of flow and macro dialing your grinder will do this.