This makes no sense to me. Why would supermarkets get involved, as they are inundated with orders and can't keep up with the demand for slots as it is?
Also, why would the web developers go to the trouble of building a site and hosting it without a fee for use? Unless it is run and paid for by the council and is exclusively for the use of shielded or vulnerable people, I don't understand it. If anyone can use it, it is not a charitable thing, so who is putting in the money?
Personally, I would be more inclined to use something like this if they charged for the service, as at least that way I could see the reason for it, but I wouldn't touch it as a 'free' service without knowing a lot more.