I think the idea is that you buy that upfront and you have much leftover for other things. The flour is $2-$3 and you shouldn't have to buy that again for a little while.
Example I would use is like right now. I buy groceries to be able to just make things. You get good deals on buying things in larger quantities.. so meats you buy and freeze the stuff you don't use.. I go weekly grocery shopping but actually only spent about $30 last week because I already had things from the last time I grocery shopped. I plan my meals out and shop off of that. I see what your mom is saying exactly. It's not an in the moment purchase, but for future meals because it's just you and you will have plenty of meals out of.. say a sack of flour. Does that make sense?
So are you saying that, let's say I only use 2 cups of flour to make chicken and dumplings. I would have the flour later to make more chicken and dumplings, is that what you are saying?
Here's another thing, you have to KNOW how to cook. If I make chicken and dumplings out of scratch, I have to know how to cut up chicken and hold a knife properly. Only my father, not even my mother, know how to cut a chicken leg by leg and thigh by thigh.