If Tacko is with the team this next year (his 3rd), then we get his Bird rights. We can then sign him to an NBA contract of any size, as an over the cap team.
Maybe being a UFA is better for the cap than being a RFA this year, as the Celtics couldn't match ridiculous offers without getting those Bird rights, first?
Bottom line is Tacko can return to Boston on minimum salary (if he wants to get a large multi-year contract from Boston next year), or he can pursue money from other teams, now. However, those other teams also wouldn't have his Bird rights, so he would be limiting himself to MLE's, now and in the future.
Therefore, it makes more $ sense for Tacko to stay for one more year, even if he were to be traded, or something, later.