I'm by no means an expert but I have participated in one or two in the past. I'm sure the nation RP people will swing by in time to give more educated opinions. But when you RP for an entire nation you are usually writing for important groups in that nation. As an example if I were writing for a nation of fantasy elves I could write about the king/queen, an army general, a soldier in one of the general's armies, whatever passes for a police officer in their society, a mayor, and a hunter that just wants to live their life. The difference between writing from the perspective of a single character and that of a nation is that the posts in a nation RP tend to be centered more around what the nation is doing and giving you an idea of what being in that nation is like for everyone. You could technically write from a different person's perspective every post, but I think most choose to write from the same few perspectives to help round out their nation's narrative and also build some interest in the characters. 

It's also not exclusively worldbuilding. When nations go to war with each other, trade with each other, or compete with each other for resources, there's opportunity for interaction and change. There are more examples I could give here but that's the gist of it. It's a bit more than just fleshing out a world. 

Not all nation RPs have you playing a nation. Some nation RPs focus on smaller groups like ships filled with pirates, corporations, gangs, things like that. Though you could also go the other way and have various worlds that are watched over by various gods. 

So the draw is that you can play as a large group of people from various walks of life and adress problems from various angles. In a normal fantasy setting I would have to play the hero, but now I can play as several adventurers, the merchants that supply them, the quest givers that pay them, and even the villains that challenge them. And how each of their operations can be changed by global politics. Imagine your nation has a steel shortage and is unable to procure more from their neighboring nations. The adventurers might be able to make due for a little while but what policies will the king going to put in place to ensure steel levels will return to normal? How will the merchants ensure they can still make ends meet? Will quest givers even want to hire adventurers who can't maintain their metal gear? Will wizards start demanding higher prices now that those with metal weapons are becoming more scarce? Perhaps foreign adventurers will become a more common sight as they rush to fill the void created by a lack of new local adventurers, unable to take up swords?