Obviously there is no official information what exactly the tokens represent but some people say ?si is a tracking token to, I also think as its a id not necessary to locate the video it has to have some other, undisclosed purpose. Better to get rid of it imo.
https://www.reddit.com/r/revancedapp/comments/17g8jll/youtube_added_a_tracking_token_to_the_url_when/