Because you might not have a factory stereo. And you don’t have to go to the dealer, you can buy a decoder for cheap, or learn your car’s konami code.
OR...you can be like me. I have a 1969 VW beetle. I spent two years restoring this car. She and I are one. I can hear, feel, smell, and sense anything wrong with the car. No need for the idiot lights.
And unlike my ex-wife, she won’t leave me for another guy.