Fit on the court
First, when you have an elite player like LeBron James that can score, distribute the ball and create plays for the rest of the team, you need a player can be good off the ball. Also, as LeBron is not an elite 3-point shooter, you need shooters that can open the way to the basket.
During his last playoff run, Al Horford was fourth in usage rate on his team, behind Kyrie, Jayson Tatum, and Terry Rozier. However, being the fourth option, he still averaged 13.9 points, 9.0 rebounds, and 4.4 assists with 40.9 percent from 3-point land per game in the playoffs. With these numbers, we can assure that on paper he could be a great fit with LeBron.
Also, if you sign a center to pair with LeBron, aside from stretching the floor if you can have him defending elite players would be a huge extra. Horford was a solid defender, being in the top 10 in centers in defensive win shares. Defensive win shares is stat the NBA calculated that estimated who many wins you can have if that player is in the court playing defense.
However, if this stat doesn’t convince you, just look at how he defended Joel Embiid in their second-round playoff series against the Sixers. And remember that before the Celtics locker room went bunkers, after the team’s first win against the Bucks in Game 1 Horford was being annointed as the Giannis Stopper.