Why for the Knicks: Pretty self-explanatory, they get their preferred superstar in the deal.
Why for Bucks: I don't believe Giannis can fetch a true blue-chip young guy anymore, but OG Anunoby is a pretty solid player, and Jared McCain is an intriguing prospect. If you think of Jared as a first, and the other two firsts, you've got three first round picks.
Why for the Spurs: They're the interesting wrinkle here. I think PG can potentially out produce both Barnes/Vassell, and it's an upside swing worth taking for them, if they want to improve the team but not have to make a deep investment like with the Giannis thing(that doesn't seem to have as much legs for them as first thought.)

Why for my 76ers:
Welcome home Mikal Bridges, and I'm sick and tired of Josh Hart dominating us on the boards. In exchange for facilitating Giannis in a Knicks Uniform, these pesky role guys are the things we need to pair next to Maxey/VJ and Embiid.

(Note: Only reason this doesn't work in the trade-checker is that Mikal can only be dealt at the deadline.)