The delta is a Greek symbol for the triangle - three sides formed together - in ancient Greece this was also a symbol for defense. The center is protected on three sides all interlocking. The star is four spears tied together at their ends, this was an ancient symbol for offense. No army can prosper over a protracted war unless it has both good defense and offense. Defense is integral to survival while offense is integral to winning. If you are protecting what you already have taken then defense is enough but if you want to expand your borders them you must also have a good offense. Both the ancient Greeks and Romans were masters of war using basic shields and spears to win time and time again. The shield is defense while the spear is offense and the two were always paired together. Eventually the Roman Empire fell to heavy cavalry. The Germanic tribes were the first to use barding on their warmounts and they wore heavy armor as well. Calvary strikes with the blunt side of their swords using the speed of their mounts to break bone and advance through the perimeter. The fall of the Romam Empire brought about the dark ages which lasted for several centuries - western Europe fell into a state of disorder.
40k is much the same to me. Every army should have both a good offense and a good defense; there must be a balance as well. Mobility is a key to success and is a fundamental cornerstone to both. The Romans eventually lost as does everyone. Their great empire fell because they were too slow to adapt to the change. Time and time again the shield and spear was more than enough then there came the day that it no longer was what it had once been. If you are successful and become set in your ways the world will change around you. Therefore it is vital to keep an eye on change and plan accordingly... Even the best laid plans will eventually fail.