I've never used one, but I think the deal is more of the color accuracy of your monitor, and then that to your printer.
I believe there are Color Management profile files on your computer that they both (Printer properties and monitor properties) can reference. Maybe referencing the same one?
In your example, your monitor may be correct, but the printer of the company you uploaded to may be calibrated different, or using another color profile than your local machine.
Maybe the spyder creates a custom profile file that both the monitor and printer can reference, I don't know.
I *believe* this is how it works. A simple Google or wikipedia may clarify.
Once I had my CRT HDTV calibrated and he used a similar professional gadget, to adjust the shades of grey. (all colors are a shade of grey...)
Edited by kenhdog (10/04/08 02:30 PM)
Edit Reason: clarified and typos
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