Linked from
The 25 pages that link to Solid angle, each with the reason it gives.
Inverse-square lawRelated: A fixed solid angle covers area proportional to distance squared.
Numerical apertureRelated: The accepted cone of rays can also be described by its solid angle.
IlluminanceRelated: A source's apparent extent helps determine the flux arriving at a surface.
Radiant fluxRelated: Radiant intensity relates flux to the solid angle over which it travels.
Radiant intensityRelated: It supplies the directional area over which radiant power is counted.
RadiometryRelated: Radiant intensity and radiance specify power relative to a solid angle.
Spectral radianceRelated: It specifies the directional spread of the radiation being measured.
Luminous intensityRelated: Intensity is luminous flux emitted per unit solid angle.
CandlepowerRelated: Luminous intensity is defined per unit of this angular spread.