I used to only buy for a specific pattern. Until it took me several years to get to the one project to use the fabric I'd bought. Then, guess what!?! I didn't have enough of one of the primary fabrics, and it was no longer available:{. Now, I buy a little extra, just in case, and that has turned into my stash. And, sometimes, the fabric calls my name, and if it does, I buy that without a specific pattern in mind, 'cause ya just never know, lol.
At any rate, because I work really odd hours, it is nice to be able to walk into the room and grab fabric whenever the mood strikes me to make a quilt, and usually, it's during the nighttime hours, when ALL the stores are closed.