It is a useful tool to have though not essential for stringing a racquet.
One example is best explained by the author at the following website.
I find it more useful as a finishing clamp rather than a starting clamp. I use it to tighten the knot rather than using a plier. Reason? It's less abrasive on the string than using a pair of pliers.
Please do note - never remove a starting clamp which is holding a tension-ed string as you can damage the area of the string held by the starting clamp eg break outer jacket of string.
