PME in-res exists across the services and not unique to the Air Force. In peace time, this is simply a rat race (as an example, no derogatory insinuation intended). At the maze's exit there is an O-10 billet waiting for the lucky rat. To get there, the rat has to successfully complete various tasks, DO, Sq/CC, PME in-res, various levels of O-6 command, exec tours, joint staff tours, inter-agency tours, SLS, Pentagon, MAJCOM command, COCOM command and etc. For most in the Air Force, PME in-res and Sq/CC are the "pinnacle", but really they are just entry level objectives in the grand scheme of things. So while you don't need PME in-res to be a Sq/CC, but you are likely to need at least one of the two PME in-res to be a GO.
Knows the rules of the game before you play. Or to quote WarGames: "The only winning move is not to play."