As a human, you want to stand up. You would rather stand straight than hunched-over, or with any partial squat. You desire to walk around using your legs. You try to walk like everyone else. These are very basic concepts.
But consider one joint. A knee. A hip. An elbow. How many bones are used in the elbow joint. The answer is three. How much range of motion does one joint have? It depends on flexibility.
Now consider how many joints you have in your whole body. Do you know how many? The average fetus has 300 bones, while the average adult has 206 bones. Your back alone has over 30 joints, and there exist infinite mechanisms in the feet. The bones fuse over time, reducing the number of "flexible joints."
It can safely be said that there are a few hundred joints in the human body. Consider the tremendous range of motion of the whole being, and compare that to the ability to stand up straight.
People who are more flexible can generally stand up straighter than those who aren't.
Now consider gait, or the way a person walks. First, standing up straight, trying to vertically stabilize a few hundreds joints; then, taking the first step forward. It's like trying to balance a tower of stacked plates. They go all over the place!
Human gait is a forward-falling, sloppy mess of vertically-stacked joints that eventually seize-up, causing the gait to slow, stutter, and then cease.
Over time, this rickety gait wears out joints in almost random selection. It certainly causes more bones to fuse, or the need to be surgically fused. You see, flexibility is lost over time and certain joints of the body wear out.
But with training, everyone can learn corrective gait and practice better walking. This lengthens the life of the joints, therefor fortifying the body and lengthening the life.
How abstruse ! But we teach corrective gait every day !