post process blend weight

What is the variable of ‘post process blend weight’ used for?

Cameras have built-in post-process settings that work in the same way as a post-process volume/component, but the effects are only visible through that camera.

See here:

The variable you’ve screen-capped sets the amount of influence for the camera’s built-in post-process settings, where 0 is none and 1 is in full effect.