The Mavs get a nice center while not giving up much. The Mavs are looking to move on from Delon Wright and Justin Jackson, so why not trade them for a dominant rebounding Center. Although Whiteside is flat-footed and acts like he is playing MyCareer every game, he does have flashes of being a great Center. This season, he has a career-high; 33 points, and gotten 23 rebounds, with 10 blocks. He has some firepower left. Also, he is a great pick and roll player, so that will be a good addition for Luka.
The reason why the Blazers do this is for two reasons. They know Whiteside is going to leave so why not sign-and-trade him to get assets. The Blazers need a backup PG for Dame. Delon Wright is a good defender and a great contribution to his team. Justin Jackson, although he is not shooting well at all.(He is shooting a career-low 29%!! from deep), he will be a nice addition to the team due to his size and he will improve his shooting while under Stotts. The first-round pick will be needed since the Mavs aren't really giving up key pieces.