okay, as an American I have to ask... is buying groceries for Christmas a normal thing in Australia? or is this even weirder than I think it is? I'm so confused.
It sounds like you think people are buying these hampers as gifts? No, not usually. These are designed mainly as pantry/freezer fillers. With expensive brand name products. At huge markups.
Remember that Christmas is about one week into our five- to six-week summer school holidays so a lot of people who get these are also stocking up for Christmas/end of school year parties, NYE, to go away camping or caravanning to the beach and just keeping hungry kids fed during their break. They don't want the effort of having to plan their shopping and stock up when they see things on sale or buy a few extra things each week or two during the year and put them aside. They don't like the idea of having to go shopping during a heatwave. Add in the (very sensible) desire to avoid the panicked crush you get on Christmas Eve because *gasp* the supermarkets are shut on Christmas Day and so everyone HAS to stock up on milk and bread to make sure they don't run out (the same happens the day before Good Friday and Anzac Day morning when supermarkets are closed).
A huge proportion of people who buy these sorts of hampers are those on government pensions/welfare (single parents, seniors, unemployed, disabled) and/or living in low socioeconomic areas. They see the pretty catalogues and don't think to do the sums versus the old-fashioned way of putting the money into a Christmas Club account. Even adding in the extra cost of buying groceries online instead of physically going to the supermarket still works out a lot cheaper.
Chrisco Delivery Day is also a bit of an event. Every year in late November/early December, a few friends and extended family on Facebook get into a frenzy because their delivery date has been advised and they've got to clear out their pantry and freezer to make room for everything that's about to be delivered in bulk. And then they try to organise swaps because there's always something in the hamper that your family won't eat.
Basically, there are those who buy these hampers because they think they are the best thing since sliced bread and think that they are only paying a "tiny bit extra" for "convenience". And then there is the rest of the population that sees them for the ripoff that they are.