What are the major evolutionary pressures for Bioluminescence?
In many cases, and in particular in marine invertebrates, the bioluminescence is in fact produced by symbiotic bacteria of the Vibrionaceae family. In most cases the bacteria can sense when they are being hosted by the animal through quorum sensing mechanisms, and start producing light. 

The evolutionary pressure for the bacteria to produce light is to be in the protected environment of the animal which often feeds them. On the animal side, there can be several reasons to host the luminescent bacteria. In many cases, the evolutionary pressure is linked to reproduction. In order to successfully reproduce in the immensity of oceans, marine animals need to know that they are among their kind before they release their gametes. 

Another example is the one of some deep see fish like the anglerfish which have developed a special "flash light" organ to host bio-luminescent bacteria. Fish can use the bacteria to emit a light to aid in camouflage, hunting, and attracting mates. See http://microbewiki.kenyon.edu/index.php/Deep_sea_fish