Why do the Rockets do it?

Adding much needed scoring and playoff experience to a young team with a KD who would be happy to renew with a team becoming contenders. The Rockets get to take advantage of an open west before the Spurs build their super-team, whilst also taking advantage of cap space before needing to pay Amen. Also get good value for Sheppard who isn't getting the minutes to develop in this team. Their starting five would look something like this:

Thompson Green Eason Durant Sengun

Still have young talent in Whitmore with a potential cheaper contract being offered to Vanvleet and a first round pick for the upcoming draft.

Why do the Suns do it?

Get to cash in on KD and retool from a side that is going nowhere. Add much needed defence and up Booker's usage knowing if you get some defense around him he can still be efficient on high shooting volumes. Get two wings who come straight in to start with the ability to Jabari Smith Jr. when his extension is due, still plenty of minutes to be shared to Ryan Dunn with his ability to play the 3 or the 4. Get a young guard in Sheppard who complements Booker nicely with his defense long-term, gradually phasing him in over time for a Beal who's contract you're going to have to eat. Get a much needed lottery pick for a strong draft so you can draft a talented big to play alongside Richards and Ighodaro. The starting 5 would look something like:

Booker Beal Brooks Smith Jr. Richards