The original reason for storing water was to ride any interruption in service.
Today, this is less necessary in many places in the UK, so people are being tempted into having mains-only systems.
The reason for supplying the kitchen tap from the mains is that it's a legal requirement - stored water is not sufficiently free of bacteria, or the risk of bacteria, to be safe to drink.
Garden taps are plumbed from the mains out of convenience - they're usually sited close enough to the kitchen to use that supply.
Today, this is less necessary in many places in the UK, so people are being tempted into having mains-only systems.
The reason for supplying the kitchen tap from the mains is that it's a legal requirement - stored water is not sufficiently free of bacteria, or the risk of bacteria, to be safe to drink.
Garden taps are plumbed from the mains out of convenience - they're usually sited close enough to the kitchen to use that supply.