Although the main benefit of having a water kit is to save water in summer, to water the lawn, veg patch and flowers. They do have some benefits in the autumn and winter months when there is a lot more rain.
Firstly, if you have a water meter installed, you will want to use the mains taps as infrequently as possible, to save money. So, you can use the water in your water butts to fill buckets to hand-wash the car, or for mopping floors, cleaning greenhouse glass panels, patios etc.
Another more communal benefit, is how it can slow the flow of water going down drains during periods of heavy rain, which can increase flood risk, and also use of sewage outflow pipes into rivers. (During high rainfall periods, these pipes are used to prevent waste water backing up pipes.)
An Olympic swimming pool holds 2.5 million litres of water. If 6,300 homes , of a small town, all installed a 200 litre water butt double kit, (400 litres total), it would save 2.52 million litres from going straight into the drain network, during a heavy downpour. With an Olympic swimming pool worth of water saved in the water butts, this could lower the risk of some local flooding, and reduce the use of a nearby sewage outflow pipe into the river.
200 litre water butt double kit, available to buy here;
Image - Wiki creative commons / Kallerna