It might be camouflage, I am just guessing, but my first assumption is that the males are showing that they have some resources to spare to develop fancy features including glowing ornaments, and the females are picking the males with the glowiest ornaments because it indicates the ones that are doing well and thus have good genes.
Male birds-of-paradise often have these patches next to stark black [or] dark plumage, so the added effect of biofluorescence may aid in making these signal areas even brighter while being used during displays,” added Martin.
For the camouflage side, they said this
In females, however, the phenomenon might have a different function. “The location and patterns of their biofluorescent plumage of many species are much more in line with its possible use as camouflage,” she said.
I'm curious what that refers to. Maybe it would look like sunlight filtering through the leaves?