Transparent calculations with uncertainties on the quantities involved (aka "error propagation"); calculation of derivatives uncertainties allows calculations such as (2 +/- 0.1)*2 = 4 +/- 0.2 to be performed transparently; much more complex mathematical expressions involving numbers with uncertainty can also be evaluated directly.

Correlations between expressions are correctly taken into account. x-x is thus exactly zero, for instance (most implementations found on the web yield a non-zero uncertainty for x-x, which is incorrect).