Let me explain:

if we would move the global entity (on the server), locally, the entity would snap-back several times as the server tends to overwrite the positon of that entity with its position oin the server, so if the client does not send the new position before the server sends his position you get the old position and the entity snaps back. So we just move a local entity and set the global entities position to our local one so we can move without lag, as the local entity will not be overwritten with any position from the server, its local after all!