Call of Duty 4: Modern Warfare- completely transformed the FPS genre, and how single player and MP existed and worked in these games.
The format used to be - the single player/campaign was the focus point, and then you threw in a couple of maps for MP that let you have some mindless fun to "keep it going".
But Modern Warfare changed the game- made MP the main event and the campaign (if its even there)- its largely just a short lived tech demo to ease you into the MP. With this, they added the whole progression system that's been synonymous of what pretty much all shooters do now that keeps you playing endlessly, giving you upgrades, new weapons/mods, skins, cosmetics, along with ranks, and gave you purpose of more than that mindless fun.
After this game dropped, what MS/Bungie was doing with Halo (up to that point, being massively popular)- simply wasn't enough for a FPS package anymore. And I truly think it's why CoD is what it still is today, while Halo is kind of struggled with its identity in the post MW landscape. They've certainly tried to apply MW-style progression in later releases, but it has never really resonated or felt anywhere near as compelling since.