Why do players of stringed insruments shake the hand that is on the strings?

Why do players of stringed insruments shake the hand that is on the strings?
That is a technique that gives the sound a beautiful vibrato or tremolo effect. It imitates the human voice that can easily produce the same effect. It is a steady and controlled rising and lowering of the pitch just by a slight amount, so that a sense of the true pitch of the tone is maintained.

You can see from the hand movement that the open part of the string, the part that is vibrating, is very slightly lengthened, making the pitch a little lower, and then slightly shortened making the pitch a little higher.