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:
To send light into the darkness of men's heart, such is the duty of the artist. (Robert Schumann)