I understand what you mean. I cant answer regarding the physiological side of things but working out your maxes allows you to plan your training in advance. You know what the goal is and you can work out backwards what you should be doing each session. If you just guess you could be way out, I suppose, either starting too heavy or too light. You could end up having far too long cycles where the SD comes too seldom, or on the other hand: starting off far too near your max.