Why each team does this:
Mavericks= They do this to pair Luka with another great ball handler. In my opinion the biggest problem with the Mavericks has been relying too much on Luka. This has resulted in Luka having tons of turnovers and having and questionable shooting nights (especially from 3). Having Kemba would allow Luka to play off-ball a bit, which would lower his turnovers and raise his shooting percentages by taking more catch and shoot shots. Triston Thomson is an expiring contract who would allow them to free up money to pay Luka in the upcoming free agency.
Celtics= The two biggest problems with the Celtics in my opinion are playmaking and paint defense. Having one of the best passers (Ben Simmons) would fix their playmaking issue. Kristaps is a great rim protector who would help anchor their defense. They also get Goerge Hill who would be their starting PG since they trade Kemba.
76ers= They do this because Jaylen Brown is a better fit next to Embid than Ben. In my opinion the best pairing with a dominant big man is a shot creating gaurd who can shoot from 3. Jaylen fits that description. Jaylen is also a good defender, so losing Ben wouldn't hurt them too much on the defensive end. They also get Powell who would be their backup center since Dwight is a free agent.
Free agent signings:
Mavericks= Andre Drummond, Kelly Oubre, Tim Hardaway jr.
76ers= Danny Green, Mike Scott.
Celtics= Evan Fournier, Kelly Olynyk, Otto Porter.
Lineups:
Mavericks=
Starting 5= 1.Kemba Walker 2.Luka Doncic 3.Kelly Oubre 4.Dorian Finney Smith 5. Andre Drummond
Bench= 1.Jaylen Brunson 2.Tim Hardaway jr 3.Josh Green 4. Maxi Kleber 5. Willie Cauleystein
76ers=
Starting 5= 1.Tyrese Maxey 2. Jaylen Brown 3. Danny Green 4. Tobias 5. Joel Embid
Bench= 1. Shake Milton 2. Seth Curry 3. Mattise Thybule 4. Mike Scott 5. Dwight Powell
Celtics=
Starting 5= 1. George Hill 2. Evan Fournier 3. Jayson Tatum 4. Ben Simmons 5. Kristaps Porzingis
Bench= 1. Payton Prichard 2. Marcus Smart 3. Otto Porter 4. Kelly Olynyk 5. Robert Williams.