Energy technology allows research into forms of energy distribution which is a necessity for other research and technologies, defensive structures, and some buildings. Additional levels of energy technology will also increase the efficiency of Fusion Reactors by increasing the amount of Energy produced.
As various fields of research advanced, it was discovered that the current technology of energy distribution was not sufficient enough to begin certain specialized research. With each upgrade of your Energy Technology, new research can be conducted which unlocks development of more sophisticated ships and defenses.