From people I know who uses them, it seems to be mostly useful because you can get things delivered to you in like one hour instead of having to have it delivered next week, which the service the supermarkets themselves offer. Some companies even can deliver groceries in just 10-15 minutes.
> you can get things delivered to you in like one hour instead of having to have it delivered next week, which the service the supermarkets themselves offer.
I just checked Sainsburys (a major supermarket in the UK for anyone who doesn't know it), and I can book a delivery slot for 6 hours from now for £4. It's not an hour, but it's definitely not a week.
I checked all the major supermarkets in the south-western EU country I'm in for the week, all give me delivery times for next week, FWIW, YMMV and yadda yadda.