One of the main reasons I do this as my friend went to a quilt retreat with me and they gave the fabric list for the quilt involved. I took some scraps and thank goodness all the time.
She and several other ladies there were not happy with their selection of fabrics (after they bought all that yardage) and she hated the piece. I was able to use 3 of my materials but at lunch time I bought a fourth fabric that worked like a charm. She just made a small quilt for one of her babies (a dog) as the dog loves it.