Ammo and food, certainly, but also other things like toilet paper, gasoline, off-the-grid supplies and equiptment...Pretty much anything and everything that people take for granted now that will not be available easily or at all when the SHTF.
[but also other things like toilet paper,]
You can wipe with a damp rag. It is re-usable. Water will be VERY important.
For survival...
Water
Food
Shelter
Ammo
In that order. Not JMHO.