What is it good for?
If you cut fabrics with zig zag scissors, they (should/can) not fray. I say should here because in theory, that is true. But satin weaves still fray as you can see in my wawawoom dress and it annoys me to no end. But generally, the fabric does not fray and you can have really flat seams that way. Which might be nice for thin fabric where a serger hem or even french seams might be visible.
Also, this is THE seam finish for historical clothes. Often used and pretty durable! And you can use them for decoration as the zig zag pattern on the side of the fabric looks cool...
What does it cost?
I got mine chep on a flea market, but they can get expensive. After Sarah Harrington's helpful hint last week, I checked Ikea and they also have zig zag scissors for 4 Euros. This is really cheap!
Who needs this?
As I said, they are not necessary, but nice to have. And for 4 Euros, what can go wrong?