What is the biological advantage of bilaterity?
Bilateria, as a clade (all animals, excluding sponges, cnidaria, comb-jellies, and placozoans) is monophyletic, and as such, originated from a single species that was bilaterally symmetrical. Sometimes the explanation for a particular trait involves ancestry, and if an innovation is advantageous, it tends to spread. Along with this group came advances in the central nervous system, and digestive systems. It is advantageous to put senses up front, in the head, with the mouth. Having a tube-like digestive system, with a mouth at one end, a gut in the middle, and an anus at the other end, turned out to be efficient. Cnidaria (and other non-bilaterians) are not nearly as diverse as bilaterians, and most bilaterians are marine. So the bilateral advantage holds both in water, and on land.