While the move sees former All-Stars Walker and Horford alter teams (the latter returning to Boston, where he played against 2016-2019), the principal angle here for both teams is creating flexibility. Horford appeared in only 28 games a year ago, with the Thunder choosing to grow younger talent instead, and ought to have a chance to contribute again in Boston. Let's break the deal for either side.
It's hard to mistake Boston for moving on from Walker, whose contract had become the Celtics' most expendable chunk of salary together with Jayson Tatum's max extension kicking in following season. The 31-year-old Walker has coped with knee injuries for much of the previous two years that restricted his availability, and has two years left on his contract, for example a $37.6 million player option for 2022-23. Noting his age and signs of physical decline, it made sense for Boston to move on now because they reorient their roster around the 22-year-old Tatum and 23-year-old Jaylen Brown. Following a first-round playoff exit, Danny Ainge stepping down, and Brad Stevens and Mike Zarren taking the lead on basketball operations, this became a plausible juncture for the Celtics to re-evaluate his roster.
The price of doing business for Boston will be ceding the No. 16 pick, which made the possibility of taking on Walker's contract more palatable for Sam Presti and the Thunder, who continue to rebuild through the draft. It is a fairly valuable selection, landing outside the lottery but somewhere in the value sweet spot of this year's draft class. The Celtics are still developing their recent draft picks in all five places and do not necessarily have to get younger, making this a fair choice to forfeit in exchange for increased optionality moving forward. The actual value of this trade will gradually hinge on which they choose to do next. Nonetheless, it's defensible, and not as earth-shaking as you would think.
The Thunder have positioned themselves to make trades like this for a reason: they believe in enjoying with the draft, they're not interested in rushing back into the playoffs, and they've filled out their roster with just as much cheap young talent as you can. It permits them to choose Walker's albatross contract at the short term, with the hope almost certainly being to help rehabilitate his worth before trading him to a different team.
Considering how diminished Walker's market value has obviously become, it's unclear if he will change teams again before next season, barring some stroke of wizardry by Sam Presti. But at full advantage, Walker is still one of the league's most dynamic ball-screen guards. While his health along with hefty contract gave him unfavorable trade value in the present time, there's certainly a possible future where he returns to top form and becomes an appealing short-term inclusion to get a playoff team. That would permit the Thunder to finally gain, presumably in the form of more draft picks. They pulled it off using Chris Paul, and will try to repeat history. Walker is not an ideal match with Shai-Gilgeous Alexander, but he does not figure to be in Oklahoma City for a long time.