I’m always reassured to have some left over! It means that if ever I run out I know I’m not going to have a problem. I’ve also been happy to have some if we have a visitor who needs a top up.I hate washer fluid. The bottles you buy are never the right size for your car, so you end up putting the dregs in the cupboard under the sink, then forgetting about it next time so you build a collection of dregs.
About 25 years ago I bought 5 litres of screen wash that came in a standard looking container but with a removable breather spout that clips into a moulded holder built into its side when not in use … functionally brilliant so it gets refilled and is used to this day!