Linked from
The 11 pages that link to Absorption coefficient, each with the reason it gives.
OpacityRelated: It quantifies the absorptive contribution to opacity along a radiation path.
Kubelka–Munk theoryRelated: One of the two material parameters the theory needs for each wavelength.
Radiative transfer equationRelated: Absorption appears as a loss term proportional to intensity.
Spectral radianceRelated: Absorption reduces spectral radiance along a path through a medium.
DichroismRelated: Dichroism is a difference in absorption coefficients for different polarizations.
Diffuse reflectanceRelated: Absorption removes photons that might otherwise return from the sample.
Kubelka–Munk equationRelated: It supplies the K term governing light loss inside the layer.