In the menu of my Canon 5D Mark II there is a way of adjusting white balance in 100 degree Kelvin increments. What does this actually do and what are some of its applications?
I'm assuming that you're shooting raw. If not, tell me, and I'll give you a different answer.
Setting the WB in the camera does not affect the raw image data, but does affect what's written into some fields in the metadata. That information is used by the raw processor, when it's in "as shot" WB mode, to set the white balance for the image. If the white balance is set manually the raw processor ignores the metadata that specifies the WB.
So what's in-camera Kelvin WB good for? If you have a color temperature incident exposure meter, you can take a reading of the light falling on your subject and enter the information into the camera's WB settings. Then the camera will pass that information to the raw processor via the metadata. Those meters were common in studio settings in the film era when they were used to find the right color-balancing filter pack, but aren't used much these days, when it's so easy to include a grey card in the first shot, white balance to it in the raw processor, and synch that WB to all the other images made with the same lighting. The advantage is reduced WB work in post.
You work with hundreds of images a day, the labor savings might be worth the expense of buying a color meter and the trouble of using it on the set. If you want to do that, I use and recommend the Sekonic PRODIGI C-500 Color Meter. It's not cheap, though. Also, since it's meant to pick the filter pack to give you the desired color temperature, using to find the color temperature of your source is a little convoluted. There may be meters that give you the illuminant color temperature (and offset from the black-body line, which I imagine that your camera might also accept -- if that means nothing to you, and you want to understand it, let me know; it's not exactly a simple discussion) more directly.
Jim