Let's say you have absolutely perfect conditions. A plant will yield X amount, never more never less.
Since we grow in a less than a perfect environment that same plant will yield less than X every time. We adjust the plant to our imperfect environment to get as close to X as possible. We top, lst, SCROG, super crop, CO2, Molassass....... well you get the idea. You're adjusting the growing characteristics of the plant to compensate for the growing conditions you subject it to.