Establishing norms is a very efficient way to surpress competive behaviour that would otherwise be very wasteful form the perspective of the whole society.
The individual is perhaps tempted to cut the line, this would of course save him time. But if everyone did so, there would be a lot of unnecessary fighting in supermarkets. By collectively punishing those who don't follow the norms, for example by giving stern looks, gossiping or confrontation, a lot of things are run much more smoothly.
It turns out, as Robin Hanson and Kevin Simler explain in #theelephantinthebrain, that the function of norms in society play a big part in why humans aren't fully conscious about the motives behind their actions. The reason is that we then can cheat (on norms) more effectively.
For example: Occasionally it is acceptable to brag or to show off, but not too much or those in our surrounding will bring us down a peg or two. Though we all want people to notice our good qualities, skills and achievements. For this reason we need to be more subtle about how we brag. We still do it (cheat on the norm of being humble), but we want to do it in a way that those around us think it was unintentional. In many occasions we don't even understand what we are doing ourselves.