The official advice is to keep several litres of water per person per day, and when mains supply is interrupted — a burst main, contamination notice or wider emergency — you need somewhere to store it. A large water carrier with a tap holds a family's daily supply, keeps it clean, and lets you dispense it without heaving a heavy container about.
It fills the gap between a filter (which cleans water) and a bottle (which carries a drink): the bulk storage and transport that actually keeps a household going for days. Empty, it folds flat and stores in a cupboard until needed.