World Champion = Main Event
This is common in pro-wrestling and especially in WWE. With the exception of a few Wrestle Kingdoms, the main event has traditionally been a one-on-one contest for the IWGP Heavyweight Championship. Despite the immense drawing power that Chris Jericho vs. Kenny Omega had, it was still not the true “main event.” Wrestle Kingdom’s card was constructed beautifully as it promoted championship matches and were organized in an aesthetically pleasing ascending manner. If this was a WWE card, the main event without a doubt would have been the Alpha vs. Omega contest with the WWE and/or Universal Championship defended beforehand. Not to say that Jericho and Omega did not deserve the main event slot, but it just shows that the importance of a WWE world title would probably be less.
A World Championship in professional wrestling is meant to act as the highest level one can reach and the holder effectively becomes the face of the company, albeit sometimes temporary. We have had WWE events where the world championship has opened the PPV rather than close it out while matches with no championship title has been the main event (WrestleMania 28 and No Mercy 2016 among others). To be the main event means fighting for the world championship. Anything less than the world championship is probably not deserving of a main event slot. Thus it is important to create a world champion worthy of the main event spotlight. Episodes of RAW and Smackdown Live can be main-evented with tag matches, grudge matches, and even secondary championship matches. However, the large PPV events such as WrestleMania and SummerSlam should 100% without a doubt be main evented with the WWE or Universal Championship on the line.