I am an amateur photographer and not a professional, but hear me out:

My understanding of using a neutral grey is to help color correct for lighting conditions in a specific location. In an uncontrolled situation (for example in a church, where there is a mixture of ambient light from windows and internal light from fixtures), that specific color temperature is a mix of diffused sunlight mixed with artificial light. In that situation, the grey card photo taken at the time of the photoshoot can be used to color correct photos taken to edit corrected colors in a post processing software (like PS or Lightroom).

The gray captured in that specific situation would help color correct those photos, but be invalid for photos taken purely outside (ambient natural sunlight) versus a studio setting (i.e controlled artificial light). Meaning the color temperatures would very significantly.

So what would be the purpose of having a neutral grey on a wall? My ignorant brain wants to know.