Why the Pistons do this: Cade and the Pistons desperately need more offensive help, more scoring. KD is still an elite scorer and is the easiest player to fit into any team. He can still hold his own defensively as well. The picks might look like a lot, but it's #21 this year, a swap option next year (that probably won't go through), a lottery protected future first and a second.
Why the Rockets do this: Its time to give up on contending. They'll never be able to actually compete with the Thunder or Spurs with their current roster, or with any moves they can make to their current roster. This nets them a pretty solid return for a player that no one in the organisation or fanbase likely wants on the team anymore, and who doesn't fit a rebuilding timeline. Ron Holland is a nice young player and Beef Stew is a solid youngish backup big that could be good in those double big lineups. Duncan Robinson could probably be flipped to another team.