Linked from
The 25 pages that link to Spherical harmonics, each with the reason it gives.
Selection ruleRelated: Angular wavefunctions make rotational selection rules calculable.
Mie scatteringRelated: They organize the angular structure of the spherical scattering solution.
Multipole expansionRelated: They provide the angular basis for multipole terms in three dimensions.
Clebsch–Gordan coefficientsRelated: They supply the orbital states often combined with spin states.
4f orbitalRelated: They describe the angular shapes and nodal patterns of f orbitals.
Legendre equationRelated: Their polar-angle factors solve the associated Legendre equation.