zeus is a space maker, make space for the carry to kill things and farm by blowing everyone up.
blow things up, that is zeus' role in dota.
semi carries are often the strongest heroes on the map at the early-midgame.
heroes like viper, TA, SF, clinkz, huskar etc whos passives and spells are a pain to deal with early, but are less relevant when you and the other team have some decent items.
heroes like void, naga, morph, antimage, pl, juggernaught etc, only get stronger, the later the game goes, and the more farm they have. their passives like bash and crit do more damage with items.
think: passive & spells with a potential damage cap, vs passives and spells with no potential damage cap.
i still think luna is a hard carry, she can break a base much easier with glaives than dusa can with splitshot. glaives bounce to a crazy range and luna attack speed is much higher. luna is much more effective in the midgame though, so you could argue semi-carry i guess. everything is situational etc.