This tool does not replace a handy up to date PMS pantone book, however it will get you in the ballpark.
Have you ever been asked to find a Pantone color equivalent to a CMYK color ? Finding what PMS colors are close to a CMYK color is a difficult and painful experience, your client may only have a CMYK color and the manufacturer ask a PMS color, how you match these requirements ?
What your client sees is often different to what you see, is there anyway quick way to convert CMYK to Pantone code or find close one ? This is a useful tool to find the Pantone equivalent for a CMYK process color without the use of a PMS color book.