My BOB has hydration system that can hold 3 liters built into it. Since I didn't really want to store water in the hydration system itself, I keep 3 1 litre bottles of bottled water in the top of the bag. The idea is that as individual bottles, they are easy to rotate for fresher stock over time. Since they are actually in the bag, I can simply grab the bag and go, without worrying about filling the hydration system or figuring out where to put bottles of water that are on a shelf near my bag. If I have some time, then I can decide to fill the hydration system in addition to, or in place of the bottled water. If I don't have time, then down the road, I can decide if I want to use the bottled water to fill the hydration system. If I do, then that frees up the considerable space that the bottles take up in the pack, gives me 3 empty plastic bottles that can be used for other things or given to other people. The hydration system is the most convenient and best balanced system for myself, but much more awkward to fill and I'm not sure how much I want to share the drinking tube with others. Finally, in my pack I have a Katydyne water filter, so I can get more water elsewhere. For me, water is to heavy and bulky to try to carry 3 full gallons worth in my BOB if I am on foot. Now if I have my vehicle, then it is another story and I have several 5 gallon containers of water ready for use. Just my 2 cents worth.