There are times when, for different reasons, NBA players who could benefit other teams are wasting away on their own teams. They aren't necessarily useless players, but they become useless to their own team.
Sometimes a player just wears out his welcome or clashes with a coach. Sometimes a veteran player is displaced by a younger player. Sometimes a player just needs a new environment.
Whatever the reason, they waste away on the bench when they could be adding valuable minutes to another team. These are useless players to their own teams who have fallen out of the rotation but who could offer something to their present teams in terms of trade bait and offer something to new teams in terms of play.