Equanimity is highly valued throughout the Buddha's teachings. It acts as a powerful support for insight, and insight in turn, strengthens this quality of ease, acceptance, and freedom.
As the arc of practice develops from releasing suffering, dukkha, towards greater happiness, sukha, we often encounter conditioning that gets in the way of opening to deeper ease and freedom. The brahmavihara practice of mudita, appreciative joy, can be a support for releasing that conditioning.