No picks involved. Just Oly and ET for Porter.
Why for Washington: Washington's best lineups last year were with Pierce playing a stretch 4. Pierce is gone now, and their first idea will probably be to run with Porter in that role. Olynyk can fill that role as a stretch 4 better than Porter can. In addition, ET gives them depth at the 1-3. As Beal and Webster suffered from injuries last year, and Wall risked overuse, that added depth will be important as they try to build on last season. Furthermore, they are still keeping an eye towards Durant next summer, and this move clears up $2.8 million in cap space for next seasson. Also, with Durant on the team, Porter's value plummets.
Why for Boston: While Olynyk has definitely performed better than Porter so far in the course of his career, Porter has shown flashes that explain why he was the third pick two years ago. His best run was at the end of the regular season, at the performance continued into the playoffs, where he averaged 10 and 8 on efficient shooting numbers with good defense. Furthermore, he's more than two years younger than Olynyk, so additional improvement is more likely. Finally, he plays a position of greater need for Boston (the 3/4 swing role) than does Olynyk. If the growth from the end of last year carries over into this year, he can be a foundational player for the Celtics heading into future seasons, while at the same time balancing out Boston's roster this year.