What's the point of unlocking a “Human?”

Heroes work differently here... in this game you can unlock new generic heroes, sorted by race, and then you customize that base hero with different skills and equipment to turn him into the type of hero you want.
So for example all humans start off the same. You may want to customize your first hero and make him like a Knight. But then you might want a Paladin also, so you'd recruit a new human, level him up, and put his skill points into the Paladin Skill tree and equip stuff that is best suited for a Paladin according to his unique passive skills, like a cape and a hammer.