This trade aims for a win-win-win situation.
Knicks' situation: If the Knicks don't win the championship this season, they might start dismantling the team a bit, but keeping it competitive, to further develop their salary cap space (get out of the first luxury tax bracket) in exchange for a better bench. The Knicks' situation is that if they want to compete, they must follow the same path as the Pacers and Celtics, both of whom are contenders for next season.
Suns' situation: The Suns couldn't have done better this season and are looking for a second option for Booker. With a solid team setup, they just need a second star, either a power forward or center. That's why, in my opinion, the best option and the one that best suits their style of play is Karl-AT.
Grizzlies situation: The team is looking for a mid- to long-term tank, and surely next season, Ja and the Memphis Bears will be finishing a chapter, so they will be looking for someone with untapped potential to build a solid roster for the future of the franchise, that Jalen Green, who will join Cedric Coward and Zach Edey as future leaders of the team.