Linked from
The 12 pages that link to Isospin, each with the reason it gives.
Quark modelRelated: It organizes proton and neutron as an up–down quark doublet.
Yang–Mills theoryRelated: Yang and Mills used local isospin symmetry as their motivating example.
KaonRelated: Kaon charge states form isospin multiplets despite their differing electric charges.
SU(2)Related: Isospin transformations use SU(2) representations to organize particle states.