Having formerly owned a scale like this and tracked trends over time, I found that the fat percentage was largely unhelpful. When I went through a diet thing to reduce my overweight saggy self, the % fat declined alongside my weight, telling me precisely nothing of value.

Apparently, if you're seriously about knowing your fat percentage, you can get weighed while underwater. Compared to a regular scale, the change in your, umm, buoyancy says something about your fat.
http://en.wikipedia.org/wiki/Hydrostatic_weighing