Could Help the WWE #3: They can do some 'spring cleaning' on their roster and not worry about condemning their talent to unemployment.

Many fans have noted that the WWE tends to keep wrestlers on the payroll who are well past the prime of their careers, such as Kane.
Kane has been a major star in WWE in the past, but he hasn't sold out an arena or increased PPV buyrates since the end of the Attitude Era. However, the company felt that his loyalty should be rewarded, so he was kept on the roster and even featured on television for many years after his in ring career had become an afterthought.
With AEW on board, though, the WWE could 'spring clean' their roster of wrestlers who no longer greatly contribute to the programming, or who just don't seem to fit in like Lio Rush, and not worry that those same released wrestlers won't be able to find work.
AEW can be a safe landing space for released wrestlers, allowing WWE to assuage their conscience when it comes time to wish someone 'nothing but the best in their future endeavours.'