Understanding a Mesh Hierarchy
The problem (if you want to call it one) with the standard mesh class is that it treats the object it is going to render as a single solid object. In the real world, though, that's almost always not the case. Take your body, for example: sure it is a single “solid” object, but the various parts of your body can move independently. How about an example?
Take your arm and hold it straight out in front of you. Now move your arm up and down. Do you notice how only your arm is moving? If you were being rendered as a single mesh, your entire body would be moving up and down, which is definitely not what you want to do (even if levitating sounds exciting).
Now, you can break it down even further. With your arm still sticking ...