This is number 3 on the carry list.
This should be obvious, a mobile phone is a great source of information and can be used to call for help.
What is the extent of the crisis, where is safe, what other danger are around that you cant see yet.
Mobile phones in some cases have Internet access which can even further widen their scope in information. Mobile phones may have stored data like maps or first aid manuals that may be very useful.
The phone can be used as a light source.
If needed the battery can be removed and short circuited across steel wool to create a spark to light a fire. if this fails the battery case can be cut open and the corrosive chemicals inside may ignite if added to flammable solids like wood or liquids like brake fluid or oil.