Mzungus in full Adventure Gear

It is always amusing to eat out and to watch tourists. Especially, when they are in groups, they usually show up in full (khaki) adventure gear, sandals, rucksacks and if they are Americans or Canadians, they carry a bottle of water, which is attached to the rucksack or any other bag.

Why do they do that? What is the point in having a bottle of (warm) water with you? At least in Kampala there is a small shop at every corner where you can purchase cold or warm drinks and usually some snacks, too. So, why are they carrying a bottle? And why do they carry a bottle if they go to a restaurant?