If you and your friends play around or even idle on the main server, other players who have the launcher open will see a player count more than 0 and would be enticed to join in.
People are reluctant to join an empty server, and that's understandable. You would be surprised, however, how soon another player joins the server after you have, and another joins after that player, and another after another; before you know it, you have a proper game going!
You and your friends playing on the main server can draw a crowd.