Today's automotive computers will try and compensate whenever there's a problem: so drivers in most cases won't even notice any deterioration in performance. Even though fuel mileage may be suffering and the engine producing high levels of hydrocarbons and other tail pipe pollutants.
Failure to have it looked at by a professional in a timely manner can lead to other systems malfunctioning and consequently leading to failure and unnecessary repairs. When the computer can no longer compensate and recalibrate for whatever malfunction is occurring, the computer will override the input and go into preset "run on" program allowing the driver to continue to drive and appear that everything is normal except for
the light on the dash.
This is where the confusion really starts, some will tend to ignore the check engine light because they feel no difference in the way the vehicle is running not understanding that a malfunctioning system or component triggered the light and is making other systems work even harder.