[dljsjr]

It's not about the signal it's about the physical display.

Displays aren't calibrated at the factory. To use an LED-backlit display as an example, not every single LED in the world is created equally. Not every LED is going to give off the exact same wavelength for the same current value.

Extrapolate this out to all the other components and this is the reason that your monitor has built-in physical controls for changing RGB/contrast/brightness values to begin with.

Calibration accounts for this.

As for why ICC profiles are used instead of just changing settings on the monitors, the OSD options usually don't offer enough fine-grained control to get things just right. Display makers are typically targeting main-stream consumers so they provide simple adjustment controls.