Why is good posture important?
Our bodies are designed to move. If we are well-aligned, then physical stress is at a minimum. If our posture is not symmetrical and well-aligned, it is mechanically inefficient and we have to work harder to fight against gravity. Everything we do is going to take more energy, causing muscular stress, joint wear and tear and, ultimately, pain. Think of it this way: if your car's tires were poorly aligned, over time they would wear out unevenly and eventually break down. It is the same with our bodies... poor alignment stresses the tissues and causes premature wear and tear.
As the popular saying goes: if you wear out your body, where are you going to live?
Watch a brief overview of The Alexander Technique and its benefits here.