A robot's precision depends on its model of itself matching reality. Over time, wear, temperature and small knocks cause the real machine to drift from that model, so a position it once hit exactly is now slightly off, and errors compound along a chain of joints.
Calibration measures the true state and corrects the model, restoring accuracy. Treating it as routine maintenance rather than a one-off keeps a robot dependable, because the drift is gradual and silent until a task that needs tight tolerance quietly starts to fail.