a grey card can be used to set a custom wb in your camera for a particular lighting setup. handy if you've got mixed light.  at college we also were taught to take a test shot with the grey card in the scene, so later we could adjust wb using the dropper in ACR.

it's also used to set exposure, as many have said. i carried one around for the two years I was at college, but after losing/destroying two of them I bought a cheap one off eBay, only to discover it wasn't quite the same grey and had a slightly green cast, so it wasn't great for WB use. it was also off a little (for exposure) compared to my original one.  after that I discovered that my camera bag had a panel on it that gave the same reading as my grey card, so I used that for a while. Also I own 2 pairs of grey pants that are handy exposure checkers, as is the grass.

and yes, the dot on the card will probably make a difference, my cheapo card (was so cool cos it was made of plastic and folded away, also had a reverse white side that was a small reflector) has a series of white circles like a target symbol, ridiculous! (cheap asian junk that we suckers buy)