 Solution

Three distinct motivations are stabilizing the weak scale, improving gauge-coupling unification, and supplying dark matter. Supersymmetric cancellation of quadratic divergences protects the Higgs field from the hierarchy problem; soft supersymmetry breaking near the weak scale keeps the residual corrections modest. The MSSM particle content improves gauge coupling unification under the usual assumptions about intermediate thresholds. Conserved R-parity can make a neutral lightest supersymmetric particle, such as a neutralino, stable and a possible weak-scale dark matter candidate. These are motivations, not guaranteed predictions. If supersymmetry is instead broken at much higher energies, it can still organize string theory and supergravity, and it remains a powerful tool for nonperturbative quantum field theory, as illustrated by Seiberg–Witten theory.