Especially When They're Green

I have a question for you to pick your brain on:

What is the main goal for traffic lights?

Sounds like a stupid question, right? But as this random pondering fluttered across my idle brain this morning -- while I was waiting at a red light, no less -- I started to realize the possible breadth of answers.

Are traffic lights for ...
- actually controlling traffic (go/stop)?
- providing order to traffic?
- getting commuters to their destinations most efficiently?
- giving pedestrians a chance to cross?
- providing an emergency override avenue?

The main gist is, what is the primary goal that traffic lights strive for? For instance, if we didn't have the stoplights, we'd still have rules (and possibly adjusted rules) to compensate for the kind of chaos one gets when a traffic light suddenly stops working -- that's not the scenario I mean.

Rather, what if traffic lights didn't exist at all, and traffic bylaws accommodated for that lacking instead? Are there any goals/functions of the traffic lights that wouldn't be made up for? Is there something particularly special about traffic lights that can't be replaced?