They have challenges in existing stages with existing weapons that give you skins for other weapons and gear with no exlusive ablilities. (gonintendo.com)
The unlockable bonus stages are quite enjoyable as they take already existing stages and add new elements that make them even more challenging. (videochums.com)
SOS stages come up with limited time challenges for existing stages but ignoring them has no consequence. (gamerassaultweekly.com)