Good question. When Guardians were first born, everyone was a “Titan”. Just a powerful, undying soldier but no special abilities born yet. Defenders of the Last City. Then the “Titans” created the “Hunter” order. A class of Guardians charged with roaming the wilds to rediscover the land and map the terrain and scout out the enemy. From there came the “Warlocks”. Dedicated to studying the Light. Warlocks discovered how to use the Light and that knowledge was passed around. Each Class uses Light differently to suit their missions. Titans became the brute strength and the steadfast walls of the survivors. Hunters became the survivalists and lone wolves, equipped to travel the unknown alone and observe the enemy, undetected. Warlocks learned to harness pure Light, not needing to forge weapons or constructs out of the Light (except in D2. They ruined that) and instead dropping collapsing stars on the enemies, channeling pure Solar energy through their body (even enough to raise themselves from the dead), or becoming the Storm itself. Then there are special cases like Lady Efrideet’s non-combatant Guardians. The ones who chose not to use the Light as an instrument of war. It’s implied that they learned how to use the Light in different ways than the common Hunters, Titans, and Warlocks. We’re limited in the game, but I imagine the Destiny universe has many different ways to use the Light beyond the three main classes.