In this trade, the Mavericks would get two players who would really help up front in Harris and Valanciunas, players who would bring some size and toughness to the 4 and 5 spots, things the Mavs were severely lacking this season. Harris can play off of Luka and Kyrie, while Valanciunas holds down the paint. This would also free up significant cap space for next offseason, with only 4 contracts on the books, giving Dallas flexibility to go after another big star or reshape the team however they like. With the 15th pick then then Mavs could look to take another wing or a big, maybe Lively, Coulibaly, or Klintman, or someone along those lines.
For the 76ers, this would give them more versatility, as they would repackage Bertans with draft assets to get a different player or waive him for cap savings and have more ammunition further down the line. With Harden potentially leaving, the roster could use a shakeup, and getting Harris's 40 mil off the books could be a first step there. THJ is also one of the best 3pt shooters in the league, shooting right around 40% on very high volume which is what you want around interior presences like Embiid.
For the Pelicans, they get to move up into the top ten in the draft where they could get a better prospect to go alongside Zion, someone like Taylor Hendricks of Cason Wallace, and they clear up the inside so that Zion has more room to work.