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why do other teams trade people that they never use before and when they get to some other team they get used and be good

You answered your own question, usually when a player isn't used on a team it means that there is no position for that player to play because its filled up already. When they are traded though obviously the team who has scouted the player has seen some talent in him and gives him a chance to play, its probably the same thing with every team sport.