Water will find the weakness. There’s science behind preparing coffee grounds for espresso, and execution of that science is the beautiful dance that only baristas know. The pressurized water in the espresso machine always finds the path of least resistance, and so we must prepare the coffee to match that nature.
The act of equally distributing coffee inside the portafilter has one goal: consistency. Much more than a flatly tamped top, there should be an equal distribution of density throughout the entire portafilter basket, and thus a consistent density throughout the coffee grounds. If you fail to do this, water will find the weakness in your coffee’s density and exploit it. It finds those inconsistencies that the tamp hides, creating channels through the coffee and over-extracting those areas. This leads to inconsistent espresso shots.
By carefully making sure that there are no week areas, water should pass through the coffee grounds evenly, leading to an equal extraction throughout the shot.
Tip: trust your eyes. If you see that the coffee is lumped up on one side, a tamp or distributor like the OCD will do little to fix that. Even it out by loosening your grip on the portafilter and tapping it lightly but firmly on the side, then go on to OCD and tamp.