At the moment, I am doing a project about the differential spectral responsivity measurement of solar cells under biased conditions based on the standard IEC 60904-8:2014. For further information about my project, you can check at my profile.
Moving to the important part, as for now I know that this parameter can be used for designing and testing of solar cells, and/or correcting for the spectral mismatch between the light source used in the measurement from the standardized reference spectral irradiance data -these two belonged to the scope of my project. Yet, I'm curious to know more whether there are other applications requiring the understanding and measurement of this parameter.
If you know such other applications, feel free to comment in this question.