"The engine control unit (ECU) is a control system that uses feedback from the sensor to adjust the fuel/air mixture. As in all control systems, the time constant of the sensor is important; the ability of the ECU to control the fuel-air-ratio depends upon the response time of the sensor. An aging or fouled sensor tends to have a slower response time, which can degrade system performance. The shorter the time period, the higher the so-called "cross count" [2] and the more responsive the system."

http://en.wikipedia.org/wiki/Oxygen_...n_of_the_probe

When the car is tuned you should be able to go without one, I don't have one anymore. But as of now it's controlling your A/F mixture.