This cvar fixes UI issues (missing UI), generally caused by outdated (and usually unsupported) graphics drivers. However it does cause a fps hit somewhere in the range of 1-5fps. This issue usually occurs on integrated graphics cards, and to a higher extent, laptops with integrated graphics cards as most manufacturers are lazy in updating drivers on their sites.
Most notably, when 3.2 was released, Intel GMA 965 users had their UIs disappear. If users couldn't update their graphics card driver, they'd have to use this cvar, and restart wow:
/console fixedfunction 1
If you've managed to update your driver after already using the fixedfunction cvar, you'll want to reverse it to bring your fps back to normal. To do this:
/console fixedfunction 0