To wash your hands, face, or whatever else needs cleaning in the field.
I carry unscented baby wipes for the occasional "Mississippi Shower".
Added: a little hand sanitizer dealie hanging on your gear isn't a bad idea either.
https://images-na.ssl-images-amazon.com/images/I/51Zw9Aya8XL._SX355_.jpgDuring one of my well-being check missions it was pointed out that people in a DA without running water may not be keeping up with their own hygiene.
That was all it took to insure we all smelled like Isopropyl alcohol the rest of the time and lather up after each contact.