For self-contained touring, you can use the bottom bottle cage to hold an extra water bottle for use when you get into camp in the evening, or as many do, you can use it to carry the fuel bottle for your camp stove so that if it leaks, you don't get Coleman fuel soaking into everything in your pannier. MSR makes a little red aluminium fuel bottle that fits very neatly into a bottle cage. Being a belt & suspenders type of fellow, I usually carry a JetBoil to make coffee or cocoa with, and to heat water for those nice Mountain House dehydrated meals. But it's also nice to have a regular camp stove and some sort of pot so that you can cook from scratch if you have to, or want to...

TOm