It's a way to communicate with the audience. For example if a performer really looks uncomfortable playing a passage or a technique, the audience would pick it up easily. Joe Satriani (elec guitar player) seems to be calm and in control all the time even though he's playing rock music. on the other hand, Pavel Steidl has his own body language and facial expressions that are intentionally funny, sometimes.
Here's an example:
Becoming a great musician is about becoming a great human being.
"The duty of an artist is to take a demon and deliver an angel" (Dr. Elahi Ghomshei)