You can get things like cheaper overnight tariffs when the demand is lower - if you have some sort of storage system - like a hot water tank - in effect the electricity company is distributing some of that smoothing function to things like hot water tanks, storage heaters or batteries.
If you have your own solar ( either direct solar water heating, or solar electricity generation ), the hot water tank is a simple, cheap, reliable energy store.
Sure capacity isn't that great - but pretty much every house in the UK used to have one, so it adds up.