Suns do this trade to get more financial-maneuverability over the next 5 years, as well as obtaining two first round picks that could end up being valueable. Keldon Johnson would also be a good addition to help with their foward depth.
Spurs do this trade in an attempt to win now. They missed out on an opportunity to acquire Kevin Durant, so acquiring his former teammate could be the next best move.
Suns lineup after trade:
Fox, Green, Beal, Dunn, Williams
Morris, Allen, Brooks, Richards, Maluach
Spurs lineup after trade:
Booker, Castle, Barnes, Sochan, Wemby
Harper, Vassell, Bryant, Johnson, Kornet