The useable flag controls the player's ability to interact with a placeable. Useable placeables are highlighted when the player points the mouse at it, and they can be clicked on, including being clicked on as the target of a spell. Useable and static are mutually exclusive, but it is possible to have a placeable that is neither useable nor static. A placeable must be flagged as useable in order to be locked, trapped, or given inventory (presumably because such things would have no in-game effect other than adding overhead to non-useable placeables).