Dark and steel were made because the 1st generation was very unbalanced type-wise. Psychic types and ghost types were practically unstoppable, bug types couldn't do much, as there weren't very powerful attacks. Psychics could destroy just about anything, and even dragonite had trouble since he had no good STAB moves.Plus fighting types and fire types weren't useful in the game since they didn't really have advantage over much, (their weaknesses far outweighed this) to counteract this, they made the dark type to block ghosts and psychics, but it was weak to fighting and bug, to balance things out. also steel served as the new defensive type, outclassing rock types (notice how it is weak to fighting and fire, but resistant to psychic. It is still evident even in the 4th generation. since the rock type was outclassed by steel, they developed some new moves and pokemon to give rock more power, and thus, stealth rock was born. It's always a goal to keep all the types balanced, my guess is that the 5th generation will have a way to defend against stealth rock.
