Why bioluminescence?

The planet of Pandora, from the Avatar movies, possesses a distinctive and wild beauty, that in many cases, is also scientifically plausible, as I elaborated in these articles: Could birds evolve from fish?, How do the Na’vi people breathe?, What would Pandora’s atmosphere (Avatar) smell like?

One of the aspects with the greatest visual impact is undoubtedly the use of bioluminescence by virtually any organism, plant or animal, on the planet.

Bioluminescence is used by numerous species even here on Earth, but interestingly, almost all of them are underwater species.
Only a handful of terrestrial species use bioluminescence, while it is used by three-quarters of aquatic species. In the sea, bioluminescence is thus the norm, not the exception.

The extraordinary fact is that we still do not know precisely why. It does not seem to be solely due to the little light in the depths of the oceans, or the time it takes organisms to evolve bioluminescence. The mystery is even amplified by the fact that freshwater marine species also seem to hardly ever evolve bioluminescence.

Taking this into account, what could be a plausible and scientifically sound reason why so many terrestrial organisms on Pandora, if not all of them, use bioluminescence?