When a cell phone overheats, why is it important to find out whether the heat is coming from bottom of the phone or from the other areas of the phone?

It is important to determine weather the heat is coming from the bottom of the phone or from a different area because if you feel the heat is being emitted from the bottom of the phone, this is indicative of a battery overheating. If heat comes from other areas, it often means the phone itself is overheating.