Hands up who wants to see AJ Styles vs. Samoa Joe get lost in the shuffle at SummerSlam?
There can't be many, and it'd be a disaster if a match as good as this one will be was stuck in the 'death spot' right before the main event. To that point, the WWE Title match itself is unlikely to headline, so it'd be best off kept away from the glut of marquee madness happening late on.
AJ vs. Joe may actually be better as the main card's opening bout. Logically, it'll get more time and more energy from the live crowd if it's on early rather than in the co-headline slot. Look at Jinder Mahal vs. Shinsuke Nakamura last year for proof of that; whilst that one was hardly expected to be a thriller, it was crippled by card placement.
If SummerSlam 2018 can get off to a hot start and showcase AJ vs. Joe for 20-30 minutes, it'll be a blessing. Other matches, including some of the better ones like the SmackDown Women's and Intercontinental Title matches, lack the first-time (in WWE, at least) punch this one does.