I usually buy most of the material for an idea I have or a pattern I want to make. Sometimes I just like the material and the companion fabrics and buy that with no pattern in mind. If what I have brought does not look like it will achive what I want then I may buy other fabrics to go with it. It all depends. Other times I just buy a fabric, just because I like it and want to build my stash.
I guess this isn't much help. It is about as clear as mud. LOL