I think maybe it comes down to knowing yourself, and understanding what the fabric means to you, and then knowing why you buy it.
Some of us like to collect fabric. We buy a piece because it is beautiful and it makes us happy to see it or own it, and we probably never want to cut that piece up anyway.
Some of us are very project-minded and buy just what we need for the next quilt we're going to make.
Then, some of us buy like crazy when there is a great sale, never mind whether we really need that fabric, it's the bargain that matters.
And some of us simply are hoarders. Having more fabric somehow makes us feel safe. (Maybe if we have so many projects we will never be able to get them done before we die... then we will never have time to die...)
Personally, I have been all four of those people at one time or another.