Before revision:  A practical application is presented, where numerical and analytical solutions for the variance of a two-factor mean-reverting model are embedded into the Black pricing framework and market calibration of model parameters is performed. It is shown that the availability of an analytical formula for the variance makes the calibration 14 times faster , thus proving the practical value of tractable and general methods to derive it .
After revision:  A practical application is presented, where numerical and analytical solutions for the variance of a two-factor mean-reverting model are embedded into the Black formulae and the model parameters are then calibrated against listed options. In particular, the analytical and numerical method are compared, showing that the former makes the calibration 14 times faster , thus proving the practical value of tractable and general methods to derive it .