Why the Suns do this?
The owner doesn't want to pay Ayton, so they trade him for Poeltl, who is cheaper and under contract for an extra year. Plus adding Thaddy Daddy to their team will help the Suns in all the minor aspects of the game. The picks are helpful too, if they ever want to trade for a big star (like CP3 last year).
Why the 76ers do this?
They get Simmons out of Philly and send him to the complete opposite of California because he's a punk ass bitch.
Why the Spurs do this?
They find their PF and C of the future. San Antonio will be the perfect place for these players to develop and become all stars themselves. This will also give San Antonio one of the best young cores in the entire league.
Why the Raptors do this?
They don't have a direction to their team. Do they want to contend or do they want to rebuild? I believe, if they want to contend then, they should trade for Simmons to raise the ceiling of their team because he's already an all star and a DPOY candidate, and, again picks are always nice.