Spot AF reduces the effective size of the AF point. With single point 'regular' AF, the actual AF point is quite a bit bigger than the little box in the VF (basically, the real points on the AF sensor almost touch). Spot AF restricts the AF point size to only very slightly larger than the little box that represents it in the VF. Spot AF is useful for macro shooting, I also use it with small birds in thickets. It's bad for moving subjects.
On the 5DIII, spot metering is always in the center (only 1-series bodies can link spot metering to the selected AF point).