Common Mode Level at Output

This question has been asked once but I am putting this up again, for I didn't quite get a good answer.
Usually the output common mode level is to be kept at the avg of the positive and negative power supplies. Any reasons why we do that?

Appreciate the reponse.

Thanks

If you place the output level too high, or too low. Clipping will occur when the input
signal is large enough. See picture below. If you place the output level too high, you will more likely run into Vdd, and cause your signal to clip. If you place your output level too low, you will more likely hit ground, causing your signal to clip.

But if you place the output level near the middle. Your out put will have the most room to swing. (the bottom case in the drawing)

This is important if you want your amplifier to be linear. This is especially important
if the amplitude of the signal carries information that you need later.

For example, in AM modulation, the information is encoded in the amplitude. So clipping will cause you to lose your data !

Or the signals going to your speaker. If they are clipped before they reach your speakers, your music will not sound very good.

Clipping is a very ugly distortion.

Hope that helps.

To have a symmetrical signal output (avoiding signal/data degradation)

not the average, but the midpoint of the effective output voltage