Reasons for doing it:

Kings- The Kings need to get good value for Fox, and this is the (near) perfect package for them. They get a good young player in Vassell, 2 promising players in Champagnie and Branham, and arguably the best mentor in the NBA. Along with that, they get 3 decently valued picks. With this trade they get the most options, with being able to tear it all down or still try to compete with the roster they have + Vassell and Paul (the first option would be much smarter, they cannot win with this core).

Spurs- With Fox's preferred destination being San Antonio, the Kings try and get him there, but they would still take other offers so the Spurs need to make a good offer. They send Vassell which hurts, but is worth it to get a player like Fox, and his contract might not be the best. They give 3 of their many FRPs, so that is not too big of a loss. They don't need CP3 anymore and Branham and Champagnie are not crucial. This trade definitely puts them in position to compete for a playoff spot, and maybe a series, and next year if they make the right offseason moves, they can become Championship contenders in my opinion.