Understanding mechanics provides the foundation for strong understanding of human movements.
Why would you care about movement mechanics? Well perhaps you have some questions about joint loading, energy expenditure, technique...and grasping basic principles may lead you to better understanding to answer such questions (and also realize that usually it is not as black and white as some may say).
For instance, why do we expend energy when walking on flat ground?
Short Take Home Message:
The need to keep angular momentum low (no rotating) constrains our choices in how we move.
Longer Explanation (Just go to "summary" if it's too long-winded)
Center of Mass
Now, biomechanics can have many levels of analysis, but we must start with the most basic, which would take us back to physics and representing the entire body as one point mass, called your Center of Mass (CM). We'll stick to 2D.