First because he balances their roster better. They'd still have 4 good front court players, but now they'd have a really high level two-way guard. They do not have that currently.
You like the Mavs better with D-White than AD? Is it a better team with D-White than AD?
G: Kyrie, D Russell / A Simons / Brandon Williams
G: D White, Klay
F: Cooper Flagg, Max Christie
F: PJ Washington, Naji Marshall
C: D Lively, D Gafford
vs
G: Kyrie, D Russell / Brandon Williams
G: Klay, Max Christie
F: Cooper Flagg, Naji Marshall
F: AD, PJ Washington
C: D Lively, D Gafford