The PlayerID system allows you to assign a unique ID for each player using scoreboards. What this can allow you to do is link entities to only one or a group of players, keep count of how many players have joined your server, and replicate /tpa command in vanilla for example. Look at my other commands that involve heavy use of assigning unique IDs to players, mobs, items, and more to make the what was once thought to be impossible, possible without plugins or mods.