If you've used a ray/path tracer you know that the output always starts out noisy - rays are very discrete things so you need to trace a lot of them if you want things like soft shadows, global illumination, depth of field and motion blur. The noise comes from the fact that scattered rays are sent towards different directions for each pixel.
But there's nothing stopping you from just, not doing that
Didn't know about this! Looking at the examples, they look a lot like the images you see while baking indirect light in Eevee, which always kind of perplexed me.
