Gain is the amplification factor. A system that relies on a gain of 5.0 may not work properly if the gain is really 5.1. Most analog circuits are well described by the equation of a line. Indeed, most precision converters have a provision for trimming both gain and offset parameters via external pots. Thermistors and thermocouples change sensitivity with age.
Photocells drift with age and temperature.
We designed a system years ago with lead sulfide photodetectors - the sensors were an order of magnitude more sensitive to temperature than to the infra red light we were trying to measure. It seems that a sensor for measuring almost any physical parameter to a reasonable degree of accuracy will, uncorrected, lead to significant errors. This is why every instrument in your lab, be it a scope, voltmeter, frequency counter, or whatever, should be calibrated at regular intervals. You should also change your car's oil every 3000 miles and call your mother once in a while. The frenetic pace of 1990's life seems to insure that equipment calibration will never be a high priority concern to most of us.