Celtics: Tatum and Brown are both great scorers, but neither are great at anything else. Brown especially struggles when forced to be a passer, which leads to he and Tatum 'taking turns' when the offensive flow breaks down and they aren't getting fast-break opportunities. Markkanen gives the team a long-term answer in the front court next to Rob, with Al and Danillo becoming the top backups. OG provides superior wing defense to Brown and is a better shooter than Smart - this makes White the starting PG with Tatum continuing much of the ball-handling duties. Brogdon would continue to be the 6th man and PP would become the backup PG, alleviating some of the log-jam. Largest concern: Markkanen/Anunoby aren't any better at passing than Brown. The Celtics will also have ~8mil to spend in free agency AND get under the tax, in order to reset for long-term extensions.
Jazz: Markkanen is a terrific young player, but they upgrade to a top-scorer (who's only 6 months older) in Brown, to lead their next contender (hopefully). Some may be concerned that Brown wouldn't resign in Utah, but he has repeatedly said that he will sign the contract when he gets it - whomever is paying him doesn't seem to be a top priority (as long as he gets the super-max). Williams is a great fit if Markkanen is traded, so this simply facilitates a restricted free agency deal to a team where Grant will have a larger role. The cost hurts, but with most of their draft-picks available still, the future looks bright in Utah.
Raptors: They've been looking for a PG and 29yo Smart, 1 year removed from 'defensive player of the year' would be a great fit - adding the 16th pick and a future swap of a 2nd for a weak 1st, are just the cherries on top. There have been grumblings that OG isn't happy in Toronto, so moving him a year before he can opt-out for a starting PG is good value on its own.
The goals of this trade:
Provide the Celtics with a better balanced team while avoiding the Brown super-max and creating cap flexibility
Use the excessive Jazz draft-pick stockpile to gain a superstar player
Acquire a long-term PG for the Raptors