Why Golden State does this: Both are expiring contracts. One can actually help their team while the other does not. Especially with the uncertain future of Carl Landry, Golden State's need for frontcourt depth is even more exaggerated now. Humphries is a great rebounder, and his high-energy play fits right in with the Warriors hard working culture.
Why Golden State doesn't do this: There is no reason. Following a postseason where Jefferson single-handedly lost them Game 1 against the Spurs, they will gladly trade Jefferson for a similar expiring contract that will actually be of use to them.
Why Boston does this: Humphries is too talented to simply give DNPs to. Unfortunately, he plays the same position as promising young talents Sullinger and Olynyk. Bringing in RJ gives Boston a guy they'll be fine giving DNPs to, but more importantly, offer a veteran leadership in the locker room. For a rebuilding team that just lost its captain and defensive identity, a presence like RJ is certainly much needed.
Why Boston doesn't do this: Again, there is no reason. This trade is a win-win for both sides, and even though it's incredibly boring, it's highly beneficiary.