I have a friend that does the same thing and it just seems really weird to me.
My groceries aren't mine until I've paid for them. And also, as a person with not always a lot of money I sometimes (very rarely, but still) find that I don't have enough money for the things I wanted to buy. What if I'd started consuming the goods, got to the till and couldn't pay?
If I needed to drink first I'd buy the drink first, drink it, and then proceed with the remainder of the shopping.
Anyway, people can do what they want. I'm not going to berate anyone for this. But I still think it's really weird.