I'm a beginner sewer and was told I would need this for all the mistakes I'd make.  Yes, yes indeed, I did.  At first it seemed cumbersome, but once I practiced and got the hang of it I was ripping seams left and right in no time flat.  It doesn't clearly explain this to dummies like me that you use the sharp end to cut individual threads if need be, but you actually put the blunt end on the bottom and the U-shaped part does all the seam cutting.  The sharp end on the bottom will rip your fabric!  The red nubby thing protects the pieces of fabric from tearing.