Because an individual pixel can be a bit too specific when looking at a wider image?
I often sanity check my shaders/materials/white balance etc in IR using the right click and looking at sRGB values.
But the individual pixels can vary wildly across the same shader depending on the light setup, resolution and how long it's been rendering for, which can give you a false reading or at least skew the reality of the final render, see attached screenshot of a white wall shader after a few IR passes. Depending on where I right click on that screenshot, I'll get readings of neutral grey, warm grey or cool grey.
Let's say I'm trying to achieve a warm grey, but unwittingly sample a cool grey (it's rare I'm that zoomed in when sanity checking), it might result in me overcompensating elsewhere - either in shader or in white balance or in lights, when in reality, those pixels might average out to the warm grey I'm looking for if I just leave the render running.
As I say it's very non-urgent but would be a nice to have. I also appreciate it's quite an anal request in terms of precision when I could just sample a few areas and eyeball it/mentally decide if it's close enough or needs work :)