Could help the WWE #1: Competition drives businesses to be their best.

Simply put, the WWE have been the only major sports entertainment promotion for so many years that they may have grown complacent.
The WWE grew from a regional territory to a massive entertainment juggernaut which is viewed on every continent on Earth save Antarctica. Along the way, they have gotten really, really good at what they do. The promotional video packages highlighting wrestler feuds, for example, rival big budget Hollywood productions based on their slick presentation and great editing.
However, video packages don't make up a wrestling show, and the WWE has been relying on the same tried and true formula for decades. Pick a wrestler (John Cena or Roman Reigns for example) and push them through the roof, no matter what public reaction is to them.
But with competition, the WWE may have to experiment with new techniques to get the attention of wrestling fans. This will lead to WWE being a stronger promotion overall, which is the intended effect of competition in a capitalist system.