Lip rounding is an important visual component in the perception of speech.
Where lip movements become really important is in the process of lip reading, which as I said, we all do. In a noisy environment we can tell from the lips whether the person said "beat" or "boot." If he rounded the lips, we know he said "boot."
And if he rounded the lips and dropped the jaw simultaneously, he probably said something like "bought."
Now, what happens if he says the word "bought" but doesn‰t purse the lips or drop the jaw?
The word comes out fine but it is definitely difficult to lip read.
If he says the word with no lip or jaw movement and at the same time vividly manipulates the mouth of a puppet, the movement will draw our attention visually.
We will perceive the puppet to be doing the talking. We have just experienced ventriloquists. Notice, that he did not "throw" his voice as many people believe.