As I understand it, many renderers use photon maps to get a rough global
illumination "solution". If used alone to determine indirect illumination,
the result can be very blotchy and artefacted as the photon maps are
usually quite coarse, but final gathering is a way to clean them up. There
seems to be a good visual demonstration (not explanation) here:

http://homepages.paradise.net.nz/nickamy/finalgathering.html

I doubt the rendertimes given there are representative of a production
renderer.

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