Doctrines are important because they help summarize the bible. This helps because most people don't want to read the entire bible, but still want to know what exactly it says.
If the catholic church had defined what doctrines were, they would be more widely popular and well known then they are now. I mean, you can't just go up to someone and say something about the doctrine, because it isn't uncommon that they don't have a clue of what you are talking about.