Where the fuel tanks are - in Hueys and Blackhawks they are in the aft cabin, but 53's have a ramp there, so they stick 'em on the outside.
They can also strap tanks inside for extra fuel, and/or they can have an aerial refueling boom.
The sponson extends out from the fuselage to carry fuel without getting in the way of internal space and the back katch like others have mentioined, but it is aerodynamically “wing-shaped” to reduce air resistance and provide some lift when flying forward. (adds to resistance when hovering, but the overall effect is a benefit for range and speed.)
The Cobra and other gunship helicopters use their rocket launch winglets the same way: They are primarily for carrying weapons or fuel tanks or IR cameras and bomb-aim lasers ... But their shape acts like wings when traveling forward at flight speed.