They're useful for neatening seam allowances on fabric that frays. I'm thinking in particular of lining fabric, where you don't want a bulky seam finish, but need to avoid long, wispy threads hanging out of the garment.
Quality of pinking shears is variable. I have a Pikaby pair which I bought years ago with no great expectations, as they were cheapish. They turned out to be good.
Am I alone in liking to use pinking shears? All the pretty zig-zags, I enjoy lining them up!