What is a Knock Sensor and Why?

[BoxMasters]

I've got a 1991 Chevy Suburban 2500 TBI 454, and I've also got a Check Engine Soon lite and a friend tested the codes and it says "49," Knock Sensor.
I'm going to change it out, but I'm wondering, Why is it there?
Does it mean there's something bad Knocking? Will it lead to something major?
If I replace it, and the dash lite goes off, does that mean it's just a faulty Knock Sensor?
If so, Why do even the older vehicles have sensors that fail? Just to get rid of old GM parts on the shelf collecting dust?

Thanks

[2000CAYukon]

A Knock sensor detects engine knock. In a computer controlled car/truck, if knock is detected the PCM will retard the timing.

You light can be on due to bad wiring to the sensor or the sensor itself has gone bad.

Older cars and trucks do not have a knock sensor since there was no computer controlling the timing.

//2000CAYukon

[BoxMasters]

I've got a picture of the Knock Sensor from the LMC Truck Parts Guide and the location on the 454 engine, but I don't see it.
It says it's next to the MAP sensor, which I csn see, but I don't see the Knock Sensor.
Can you tell me what's it connected to, or where on a 91 454 it would be located?
Thanks

[2000CAYukon]

I have the 90 Service Manuals and it shows the knock sensor in the block near the starter. It occupies a freeze plug hole in the block.

//2000CAYukon