@AnnieSnap and @Thefroglover
I think that people who buy calendars are people who would be buying beauty products anyway.
Things like: a nice serum, moisturiser, cleanser, SPF, retinol, mascara, blemish treatment, lip balm, candles etc are things that I would buy anyway, and it's usually much better value to buy as part of an advent calendar than to buy all these things individually.
I also like to try new products, and have discovered lots of the products I love through advent calendars.
If I buy a few good advent calendars and stock up on the products that I need, then I tend to buy very little else throughout the year.
Obviously you need to pick your calendars carefully to make sure that they actually contain the things that you want though!