Al said
"true but if you want fuse wire, or a hot water bottle, or a bottle of whisky cos a friends popped by - you need it there and then. no use getting online to Amazon."
but actually it's a lot quicker if you want to buy - say - elastic, to go onto A than find a shop that sells it (especially if you live in rural England where there are shops 10 miles away in any direction, but who knows if they will have what you want.