Everything you shoot in Halo 4 will contribute to the development of your character. Whether it's a competitive Slayer match, a cooperative mission you just downloaded in Spartan Ops or the middle of a campaign level, Halo 4 is using the spaceship Infinity as your hub to keep track of everything you're doing right.
Think of Infinity as a kind of giant 3D menu where you will swap out different kinds of armor and reconfigure your weapon loadout. Except this menu also serves as a kind of level in Halo 4 where eventually you can walk around and blast things because you simply can't tolerate enemies all up on your spaceship like that. This would suggest that things go horribly wrong down on the Halo ringworld (gott have place to drive those vehicles right?) itself and so battling on Infinity will probably be extra desperate. Just the way we like it.
This trailer with a fancy announcer and dramatic music does a great job of explaining what we're talking about.