Way #2: Allows Wembanyama to play power forward to start his career.
Although Wembanyama is eventually expected to play center in the NBA, he probably won't do so full-time until he's physically ready. Re-signing Poeltl would not only give Wembanyama his entire rookie contract to bulk up, but also give them someone who can defend big centers in the meantime. Better yet, he has plenty of experience with the Spurs, having previously played five seasons with the team, meaning that there wouldn't be a learning curve or risk involved with bringing him back.
In fact, assuming the Spurs bring back Tre Jones and don't find an upgrade at point guard, they could trot out a similar starting five to the one that started last season. Jones, Devin Vassell, Keldon Johnson, Wembanyama, and Poeltl would make for a potentially very good starting five, albeit with a potential flaw.
Of those five players, only Vassell shot at least 35% from three at a high volume last season. That's not to say that Jones, Johnson, and Wembanyama can't shoot, but Jones hit just 28.5% of his threes last season, and Johnson has been incredibly streaky since entering the NBA. As for Wemby, although he looks the part of a 3-point shooter, he only hit 27.5% of his five attempts per game last season for Metro 92.
Some of that was shot-selection-related, but there was also a shorter 3-point arc. However, with him there to draw attention away from the team's perimeter players, Johnson's shot comes back around, and Wemby himself may benefit from playing with better teammates. If that were the case, then that lineup would be fine offensively, not to mention how good it could be on the defensive end, but more on that shortly.