Why is static equilibrium important?

is the situation in which the net force, and the net on an object are both zero. It is important in many practical applications, including building design, and bridge design.
In order for, say, a bridge to have and maintain its desired shape, all of its parts need to be in static equilibrium in the absence of external forces.
Additionally it is important in most contexts that the static equilibrium also be stable. A stable equilibrium is one in which perturbing inputs tend to increase the potential energy of the system, so that there will be a tendency for perturbations to relax back to the stable equilibrium.