To calculate the radiance for a spectral region with a finite width (e.g., to estimate the Earth's energy budget or simulate an instrument response), one has to integrate this over a band of frequencies (or wavelengths).
Here, the transmission in a region in a band is characterised by a set of pre-calculated coefficients (depending on temperature and other parameters).
Radiative transfer models are also used to optimize solar photovoltaic systems for renewable energy generation.
[1] Another common field of application is in a weather or climate model, where the radiative forcing is calculated for greenhouse gases, aerosols, or clouds.
One such project was ICRCCM (Intercomparison of Radiation Codes in Climate Models) effort that spanned the late 1980s – early 2000s.