Linked from
The 18 pages that link to Emissivity, each with the reason it gives.
AlbedoCompared with: It concerns thermal emission, not the reflected fraction of incoming radiation.
Radiative coolingRelated: High infrared emissivity helps a surface shed heat radiatively.
Spectral radianceRelated: It sets the strength of thermal emission that adds to spectral radiance.
Black bodyRelated: It quantifies how closely a real surface approaches black-body emission.