To be quite frank, I have no clue. Sometimes though, you just need to make something, to fulfill a deep desire in you to craft, and when that time comes I want a yarn that fits that mood. I have yarns that are cheep and silly, yarns so expensive I have to wait to find the perfect project for them, and some yarns that I've been given from friends that I don't want to use just because that would change the nature of the gift for me.
Right now my stash is hovering around a boxful, and that is about enough for me. But it fluctuates.