UPDATE: The reason that RAW had a different format last night (with the Brock/Cena segment in the middle of the show and a Mark Henry/Rusev segment in the main event) was due to the competition from Monday Night Football. The Brock/Cena segment was placed towards the end of the second hour as an attempt to get fans to watch during halftime of the NFL game last night. That’s why Cena kept saying “halftime” during his opening segment with Paul Heyman. They were trying to tell any fans that might tune out for football when they could come back and see Brock and Cena.
The way that Heyman and Cena were used in the first half was a way to counter-program against the NFL based on the minute-to-minute ratings when Lesnar, Cena and Heyman have interacted over the last month. WWE felt that if they made an ongoing story-line through the first part of the show, they could get the audience to stick around or at least switch back and forth.
The show was set up to “peak” with the angle, followed by Roman Reigns vs. Seth Rollins as the main event match and Mark Henry and Rusev as the final segment, to build to their respective matches at Night of Champions.
ORIGINAL: Monday on RAW, WWE booked Mark Henry and Rusev in the main event segment to promote their Night of Champions match. The company did this because they were trying to save their “money” segment between Brock Lesnar and John Cena for the halftime of the Monday Night Football game. The company specifically filled fifteen to twenty minutes with commercials, backstage segments and promo spots so they could stall in order to get to halftime.
The company generally draws their best ratings in hour two of RAW, particularly during football season while fans switch back and forth. The idea was presumably to put their strongest segments and matches in those high-profile spots to get the most out of viewership habits and to promote Night of Champions.