I buy stash (usually on sale) because I make quilts from my own designs much more often than I copy others or use a pattern. I can choose to make a pattern designed around the prints I have on hand.
If I start drawing something at 6pm and then at 8pm just *have* to start making it right away, what would I do if I didn't have my own fabric store of sorts to shop from?! Just like I keep extra blades, or needles, or thread, or rulers, or any other part of my *tools* on hand, I keep "extra" fabric in a stash.
By having a stash, I have recently made a curved log cabin quilt using 30 fabrics in prints of aquas, red, and yellows ALL pulled from my stash, even the backing. I also made a reds, tans, and navy rail fence lap quilt all from stash.
Oh, yes, I'm ever so thankful I can shop in my own fabric store any hour any day of the week!!
Jan in VA