The C's may do this because Wall has the potential to be better than Rondo, and McGee is the center they need. Nobody wants Blatche and Lewis's terrible contracts, but the C's can afford to do this because they have $40 mil in expiring contracts coming off the books this year.
The Wizards do it to get rid of Blatche and Lewis primarily. It would be essentially a salary cap reset for the team. Additionally, they get a young all star in Rondo to build around. Ray Allen could be flipped to another contender for a pick or young players.