I find it pretty interesting how similar this looks to real-life film grain, which appears only in real film and not digital photographs. Here's a great article I found explaining how this effect works in film: https://www.richardphotolab.com/blog/post/film-grain-and-pixelation
Following gavin's comment about how similar this looks to film grain or digital image noise, I would imagine that getting noisy images because of underexposing a photo could be an analogue of the undersampling we see in this slide. One way to think of this could be that a longer exposure time or a larger aperture allows more rays per area on the sensor, or more samples per pixel.
This shows that path tracing is a good analogy to real film shooting. As more samples per pixel taken, the image is more and more close to the steady state (the real world).