CD players, like any other modern device with complex moving parts, can suffer damage and end up not working. Whatever the reason for it, sometimes the laser in your CD player will end up in the wrong place, or on the wrong power level for your purposes. If the player is calibrated, these problems will often go away.
Correct Track Detection
A misaligned laser or potentiometer in a CD player can render the device unable to access the portion of the CD that contains track information. This will, in turn, prevent the CD player from being able to determine where each track is on the disc, so the CD will not play. Recalibration brings the laser and potentiometer back in line.
A misaligned laser will have problems reading data from CDs, particularly modern CD-Rs that are burnt at home using a different kind of disc than store-bought music CDs. Recalibrating the laser, which potentially includes adjusting its power level as well as its "contact angle" with the rotating disc, will often return the CD player to normal working order.
Extending the Life of the Player
In many circumstances, a CD player that appears to be broken is simply misaligned. Lasers can lose power, or become loose. The CD player could get knocked and the laser get shifted as a result, which changes the angle at which the beam hits the CD. Readjusting the laser will often result in any problems in reading or playing CDs being resolved, extending the life of the player considerably.