Well I use it mostly when doing the clouds stuff in B/W so when I Select > Color Range then use black as the color choice...the Fuzziness also selects all other colors visible within 200 (something) of solid black. This, then, selects all of the darker grays and some of the medium grays (but not all the way up to 50% gray).

I'm guessing that the 200 something is a range 200 above and below the color code hexadecimal. So if there are other colors showing, say greens and blues, some of those might get selected as well, given they are dark enough. So when using Fuzzy Selecting with colors you have to set the Fuzziness down to avoid including things you don't want.

What would really be useful is a map of the color code hexadecimals That way I could test it out for accuracy.