Why Bring one? This is very useful for little side trips and carrying the ten essentials. It also doubles as a bear bag in bear country. You put your food in two day packs and you tie them together and throw one over a branch and then pull the other up.

What kind should I get? I use a beat up little nylon bag that happens to have two arm holes and it works great! just any old day pack or Fanny pack will work.