Trev's got it spot on. I totally forgot about about buying ammo packs. Will totally save your butt in a tight spot. Other than that and upgrading armour it doesn't really do anything for you as there's no in-game economy.
Also, yes, the msissions are very, very samey. It's just more about the epic feel to them when the more you progress through the later levels. You'll replay each mission more than you'll care to think about, but different modifiers can make a map feel and play very differently