I think Ainge made the trade for a few reasons
1. Adds a 7.7 mil trade exception. I never thought much of these and they arent always valuable ( I believe Golden State let a 8 mil + one expire) but Danny turned the Pierce one into Zeller and Thomas so this could grab us a decent piece or two this summer
2. Adds some front court depth, with Sully down we are very thin at the 4/5. Jerebko can come in and give us 15-20 minutes a night.
3. I believe when Prince came, Danny talked to him and said effectively: "If you play hard for the next month, we promise we will either move you to a playoff team or buy you out" As Prince was a model citizen Danny had to move him or else he could go to the Clips which would hurt the Clips pick
3b. As someone mentioned, Ainge has earned a lot of capital around the league with agents and GMs, he has moved most of his guys to contenders/ good places for them and has helped out several GMs in the process.