Celtics are in need of a couple things, a starting point guard of the now and future. The second will be looking for a third super star either now or in the future and this sets up for the star next year.
Rubio can be the starting point guard for day one. The second is by having Rubio it gives us a solid back up point guard once the draft pick is ready.
With the number 4 pick in the draft, Celtics would taken Jalen Suggs. Suggs honestly I think could be the rookie of the year, specially on this team as he could start day one if needed. Suggs gives the Celtics a starting point guard and on a rookie deal.
Baynes gives us another starting center to go with Moses, Horford and maybe bring back Fall. Also Horford is able to play the four if needed where he wants to play.
Toppin wasn't a good fit with the Knicks with Randle's coming out. Toppin with the Celtics can either be a starting power forward, if you want to moves Tatum to the small and Brown back to the shooting. Also Toppin can be a small ball center or a good bench player to come with energy with a player like Nesmith.
This year it keeps the Celtics in the race for these reasons, plus able to add another player or two in the rotation with the trade exceptions. The best part of this trade is next year with letting Smart go and stretch Horford contract you can sign Beal. I think even with the money now with Rubio, Baynes would be off the books, I think you could keep Smart on a decent contract an max Beal. Plus Suggs becomes the starting point guard, Beal at the shooting, Brown at the small and Tatum at the power, Moses becomes the starting center.
Timberwolves do this for a draft pick and some help in the front court. Culver hasn't been a piece that works with other players stepping up, so the draft pick gives them a chance to find a better fit.
Raptors do this as they get three young talents, Williams being the center piece as well as 2 first round picks. Baynes gets to go back to Boston and the 4th pick is Suggs, which they don't need another point guard.
Knicks do this trade as they get Fournier a starting shooting guard to put with Randle and the rest of the young talent. Williams gives them a true back up power forward behind Randle and not letting Toppin waste on the bench.