Sorry if I'm stepping on your toes by answering posts in here...but it's fun.

*edit* dang, you beat me to that one.
Bouncing is a great technique for a couple of reasons. Firstly, it changes the angle of the light, so that it's not coming from the same direction as the camera. This lets the light 'model' the subject and gives them depth and dimension (that is also why off camera light is so good). Also, bouncing the light effectively increases the size of the light source, in relation to the subject. So instead of the light being the size of the flash, it's the size of the reflection off of the ceiling. A bigger light source makes for softer lighting, which is usually beneficial for 'better' looking shots. Lastly, the light might be bouncing off of several surfaces and 'surrounding' the subject, getting rid of harsh shadows...this could also be called soft light.