A scratch groover is used to cut shallow grooves into leather. Essentially a simplified version of the stitching groover, the scratch groover can be used free-hand to make grooves in leather. They can be used to prepare for stitching, cut decorative grooves, mark light cut lines, or remove leather material at a fold or crease point.
A separate scratch groover is likely not necessary, if you have a stitching groover with a removable edge guide, as without the edge guide, the stitching groover acts just like a scratch groover. If you find you primarily use scratch groovers, and prefer an array of sizes easily available, one or more scratch groovers might work well for you.