Using one eliminates the repetitive motion using scissors or snips which is very bad for your hands. You aren't using your hand to cut, only holding the fabric.
I have seen the ones made with a seam ripper sticking up and they are dangerous. A guild member went the cheap way and got impaled for it. LOL If you have an unused thread cutter pendant, that is safer to stick in a piece of wood or spool.
I have seen two sizes, short and tall. I like the tall one the best. They aren't that expensive.