In Photon mapping, what 'Show Map' button is useful for?

When 'Show map' button is enabled, it shows the radiance map created by photon hits. This option is useful to discover low-frequency noise patterns and gaps on the photon map. This information isn't relatively important in still images, since Final Gather will be able to smooth and average most of the noise of a photon map, even with low-quality photon settings.

However, to avoid noticeable flickering on animations when using Photons + FG, it is very important to have a well-balanced noise-free radiance photon map. Use radius settings and increase the photon count to cut low-frequency noise in photon maps.