I have more fabric than I could sew in 3 lifetimes. Sometimes I buy FQs when I need a touch of a certain color that I don't happen to have, or when I want a big assortment of one color.
I recently bought FQs for my daughter's aqua and chocolate quilt because I don't love aqua and don't have much of it. I didn't want loads of leftovers and she wanted a scrappy quilt. The FQs fit the bill!