The leading manufacturers have taken the options for Irons beyond a simple players club or game improvement iron. They now offer variations in between, increasing your choice. That means that you need to take more care when purchasing your clubs and we need to evolve the fitting process.
A fundamental part of the PGA apprenticeship is learning how to fit golfers both from static measurements (your dimensions, posture etc) AND from dynamic measurement. The dynamic measurement is critical. Static measurements give us a base, but watching a golfers swing, measuring swing speed, tracking ball flight (trajectory and shape) and assessing ball striking capability enables us to fit precisely the solution that will be right for you.
The process of fitting to a solution that works for you has also changed. Whereas in the bygone days we would (and are properly trained to) alter the club head to create the perfect trajectory and a more consistent shot shape, there is now, because of the range of options each manufacturer offers, an additional step in the process.
These different models each create quite different characteristics to alter ease of launch, launch...