A lot of contenders are in need of an outfielder who can hit
There are two sides to any trade market: supply and demand. We have already discussed why the supply of outfielders that both 1) contending teams would want and 2) are actually going to be available at the deadline is decidedly low. The number shrinks even more if you are looking solely for 2023 rentals. However, it certainly helps that the demand amongst contending teams for outfielders is also particularly high this year.
The Houston Astros are in a pretty decent spot in their division and while they are prioritizing adding a starting pitcher at the deadline, they are in real need of another bat in the outfield that won't break the bank. The Phillies seem to have a lot of DHs on their roster and a guy like Duvall who hits for power and isn't embarrassing on the defensive end would have a lot of value to them.
The list doesn't end there as the Mariners, Guardians, Yankees, Twins, Marlins, and Brewers are all teams that are in contention that could use an outfield bat. Sure, a trade to the Yankees isn't going to happen because they don't deserve anything nice, but that is still a very healthy demand for a guy like Duvall even without the Yankees in the mix.
So Boston has a guy that is on a cheap one year deal that is an offensive producer in a market that is low on outfield bats and high in teams that need them. That is a pretty sweet position to be in if you are the Red Sox.