What is the need for TCAS? Isn't it the job of the air traffic controller to assign altitudes and make sure airplanes do not collide in mid-air? Is having TCAS mandated or optional? If it is a mandate, then is there a mechanism to notify other airplanes that TCAS is off?
TCAS is part of a defense in depth strategy to reduce air accidents. ATC does not cover all areas, so TCAS adds protection where ATC coverage is missing. ATC is not infallible: humans can make errors and computers can go down. With TCAS pilots can get life-saving warnings even in zero visibility, without reliance on any ground based system.