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The sign-and-trade is relatively common when stars leave a team that they've been with for a long time. I don't understand why this practice exists. Take Lebron going to the Heat, for example. He was a free agent. He could have gone anywhere, but he chose to re-sign with the Cavs so that they could trade him to the Heat and get draft picks in return. Seemingly, this weakens the team that Lebron had just signed with, as they had weaker drafts in his first (two?) seasons there than they likely would have if he had signed with them outright.
Why does this practice exist?