By including this hint in _NET_WM_SUPPORTED_LIST the Window Manager announces that it performs reasonable window placement for all window types it supports (for example centering dialogs on the mainwindow or whatever handling the Window Manager considers reasonable). This in turn means that Clients, when they detect that this hint is supported, SHOULD NOT abuse or often even use PPosition and USPosition hints for requesting placement. In particular:
USPosition is reserved to be used only to indicate that the position was specified by the user and MUST NOT be used for anything else (see ICCCM section 18.104.22.168 for details)
PPosition SHOULD be used for for specifying position only if a specific position should be used. Position SHOULD NOT be specified for "default" placement such as centering dialog windows on their mainwindow.
Rationale: Window managers can often perform better placement (that may be even configurable) for windows than the application. However at the time of writing this it is problematic for Window managers to decide when to use them because many applications abuse positioning flags and/or provide unnecessary default positions.
Note: The property is not used anywhere else besides being listed in _NET_WM_SUPPORTED_LIST.