Its way more exact in penitration of the needle. I have been working with springs on some machines and it makes for an easy return spring mechanism. Getting return is cruicial to the performance of your machine. I love coil springs in my machines but trying to cram a whole bunch of parts in a small space is nerve racking. The simple flat spring makes it simple to repair when its broken. Suppose your machine builder died, how would you fix it? Simplicity is the trick, just like an AK 47 sloppy as hell in side but will function long after the owner ia dead.