Most people who have been sedentary and are deconditioned have poor balance and posture. This can lead to improper joint mechanics and ultimately, injury. Balance, structure, and posture are aspects of exercise that most people overlook when they enter an exercise routine without proper training. Sure, someone can walk into a gym, strap on some weights and go to town, but it's not the most efficient or safest way to train. Even someone who has been actively training with resistance for years can lose their stabilization if it's not maintained. We must always work stabilization to ensure our foundation is solid.
To put it another way, you wouldn't put a 200 pound weight on an old, rickety table and expect it to hold up very long. The table would lean, develop stress fractures, and eventually crack. The same thing could happen to your body without the proper joint mechanics and stabilization. While risk of injury is the most important concern, you are also likely to plateau in the amount of resistance you can withstand. Stabilization allows you to support more load and delay muscle fatigue which results in more efficient and effective exercise. We need to work from the inside out, not the other way around.