I’m visually impaired (Legally blind in one eye, legally “low vision” in the other. (Legal definition of low vision: vision correctible to no better than 20/70) and haven’t had binocular vision in many years. (FWIW, my ophthalmologists say “binocular,” not “stereoscopic.”) Being monocular affects your sense of balance. For instance, most people have more trouble standing on one foot if their eyes are closed. I have more trouble doing so with my eyes open. I do not, however, rock or sway, either sitting or standing.
I’m not sure why some blind people do this. It’s certainly not universal. Most people think of Stevie Wonder and Ray Charles, two blind musicians who swayed; however, when I’ve seen blind musician Jose Feliciano on TV, he did not sway. For those who do, it may be a form of self-stimulation due to the lack of sensory information from the visual cortex. I don’t think it’d have anything to do with trying to perceive the space around them because if that were the case, it wouldn’t be repetitive.