Calibration ensures that all devices conform to an established state or condition, often specified by the manufacturer.
Calibration really makes a big difference in monitors.
Calibrating a monitor adjusts and corrects the monitor's gamma, white and black points, and color balance. Calibration software is used with hardware to send a series of colors to the screen, and the instrument reports back the value of the colors that actually arrive there. Profiling software then builds a corrective profile that is used by ColorSync to drive the monitor. Monitors should be calibrated on a regular basis. Room glare and natural light from windows and skylights can be as much a problem as an uncalibrated monitor.