From Macross Compendium
A variable fighter is a fighter that transforms into two or more modes. It usually has a humanoid form called Battroid and an in-between mode called GERWALK (Ground Effective Reinforcement of Winged Armament with Locomotive Knee-joint), in which the fighter has a pair of reverse-articulated legs extending underneath to enable VTOL.
The reason behind the development of the variable fighters' Battroid mode was the discovery of the existence of the Zentradi race, a giant humanoid race created by Protoculture for proxy warfare. Although at the time the humans had not met any actual Zentradi, the discovery of the SDF-1 Macross and its accommodations for humanoid giants justified the development of a weapon that could enable mankind to fight the aliens in land and aerospace combat. Thus, research into Overtechnology culminated in the development of the variable fighter for the U.N. Spacy.