Titans were like the primary foot soldiers of the tower. They'd be stationed to defend or attack various positions and are the most formidable in frontline combat. Hunters are the scouts of the guardian ranks. They go forth into the wild and collect information to relay back to the Warlocks and Titans. Warlocks are like the scholars if the guardians. They gather relics and ancient artifacts and study them and often utilize their technology in the field. I'm not saying that Warlocks and Hunters can't handle themselves in a fight because they certainly can. I'm just saying that Titans are the most combat oriented class. Thus seeing titans defending key outposts and positions would make the most sense. Hope this helped.