I think hatha is similar to the concept of yin and yang in Chinese philosophy, where opposites complement one another to make balance. "Ha" meaning sun and "tha" meaning moon. These represent opposites just as night and day does in yin and yang.
The main goal of yoga is to bring back into balance our imbalanced bodies, minds and emotions. Daily stresses, bad habits and poor food choices wreck havoc with these and practicing yoga can bring them back to balance.