Significance
The shape of the universe, written down. Assume only that space is homogeneous and isotropic — the same everywhere, the same in every direction — and this is the *only* geometry allowed; that uniqueness is a theorem, not a modelling choice. Everything else in cosmology follows: feed this metric to Einstein's field equations and the Friedmann equations fall out, and the redshift-distance relation with them. All the physics hides in one function of time, a(t), and one number, k.