The value is calculated from the data, as the average noise level within a frequency range a bit outside of the actual backscattered spectrum. Though the noise level might change slightly between scenes, it's expected to remain close to the theoretical -21 dB estimate. Currently, though, the values found in the metadata field seem to be gibberish - no clear pattern to the wrong values.
Scientists often use the standardized sigma-naught values to classify image features.
For example, one might classify the sea ice types within a radar image by the associated sigma-naught values.