WB Shift is pretty much only of interest to JPEG shooters.
I use WB Shift to tweak my Auto White Balance. Left on its own, in daytime conditions (whether sunny, cloudy, or overcast) the AWB will give me pictures that are too cool and a bit too green. After making dozens of test shots, I determined that an A3M3 shift gives me neutral results when using AWB in daytime conditions.
In home incandescent lighting, I use a B6M3 shift along with the Tungsten WB setting to get slightly warm results. A B9M3 shift along with Tungsten WB gives me fairly neutral results, but I generally prefer to keep some of the warmth in indoor shots.
The WB Shift can be used in general to warm up or cool down your images. Some people simply prefer warmer or cooler images, and WB Shift is an easy way to achieve that in JPEG.
Play with it. It doesn't cost anything to take digital test shots. Maybe you'll find some settings that you like.