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Presumably because it is Cool Research (TM). It's not useful, since the cost increases quadratically with the number of triangles. Which is why they only had 4096 per scene.

This will probably have some cool non-obvious benefits.

For instance if the scenes are a blob of input weights, what would it look like to add some noise to those, could you get some cool output that wouldn't otherwise be possible?

Would it look interesting if you took two different scene representations and interpolated between them? Etc. etc.

Another comment says this is faster. Global illumination can be very slow with direct methods

As others point out it's a biased comparison. Their compared Blender render ran more than 10x as many cycles as usual, ran on a GPU without raytracing acceleration which could make it slower than consumer models, and potentially it also included the startup time of the renderer.

Considering their AI achieved about 96% accuracy to the reference, it would be more interesting to see how Blender does on fitting hardware and with a matching quality setting. Or maybe even a modern game engine.