Depending on which Normalization mode you have selected, painting smooth skin weights can create small weight values, for instance, 0.008, on many skin points. For example, with the default Interactive Normalization mode, Maya can create small values as it normalizes total skin weight for each point to 1.
These small weights have no discernible effect on the skin, but they can slow down processing. Pruning these small weights can speed up processing time.