each camera and each lens is not set to an exact point, but rather to exact tolerances. As such, you may get a camera and lens combination that is perfect, but likely as not, not. Think of it this way. Say a the camera is within spec but at the low extreme, and the lens on the other hand is within spec but at the high end of the extreme. Although both within spec, together, they will not produce an optimal pic.
Even if you get good pics without calibrating, you may not be getting the best that your most excellent equipment can deliver. That is why the AFMA is offered.
I lost confidence in Canon doing the calibration when I shipped my gear to them for calibration prior to a photo safari in August. When I realized that the pics were at all focused well, I had to make adjustments in the field. Much better and fortunately, did get some keepers. Sadly, lost some great, never to capture again pics.