What HH said. The Celtics are clearing cap space. I think Ainge is resigned to the fact that Kyrie and Horford are gone, so he's resetting the deck to clear room to add young, attractive free agents that are more in the age wheelhouse of Tatum and Brown. He's definitely go something up his sleeve for free agency. Could be a big splash out of left field, or could be as simple as adding two guys like Russell and Julius Randle.
As far as the pick, Ainge didn't want to add 3 first rounders to next years team so was always looking to move the pick. Yes, it will most likely be a late first round pick coming from Milwaukee, but Boston will have their own pick next year, Memphis' pick (top 6 protected) and this pick from Milwaukee. It's really just a game of checkers, they'll probably end up trading the Milwaukee pick at some point anyways.