This is just a guess... not all quilt shops will carry all cuts of fabric. So, she may want a specific line of fabric, but can only find yardage of it. That seems to happen to me a LOT! Instead of ordering it online and paying for shipping, maybe she's simply buying yardage. Also, precuts tend to be more expensive.
Or maybe the pattern doesn't necessarily "need" variety. For instance, a d9p looks very nice with charms OR with yardage. I think a lot of times people write patterns for charm squares because that's what they have on hand, and quilters like them. The charms will give you a scrappy look, whereas yardage will give you a matchy quilt - depending on how many fabrics you use, of course.
