Chris Dillow asks a good question.
After all, most sensible Britons are localists now. Confident that significant amounts of power currently in the hands of the central state should be handed back to individuals or local communities. On the other hand, the tranzis want to remove substantial amounts of authority for 'bigger' problems out to supranational institutions (a mistake, in my view, but that's beside the point of this post). For what activity is the nation state suited? Are we just clinging to them out of status quo bias?
My answer would be that nation states are the best ultimate guarantor of individual rights.
Nation states are better at that task than local communities. The relatively dense social networks of local communities - while an asset in other areas - make them too easy to bias and sway unfairly. By contrast, as nation states are bigger even if one dense social network does take over (a family, for example) there is more likely to be a sufficient number of other people who can control them. Even when Britain was ruled by an aristocracy they were kept in significant check by the knowledge that there was a great mass of Britons out there who could only be pushed so far. A genuine tyranny of the majority usually involves too many people to really hold together.
Nation states are also better at defending rights than supranational organisations. Those organisations lack legitimacy as they lack history and have, instead, been superimposed on better established communities. A nation state's legitimacy is rooted in its history and, usually, a common stand against some adversity (wars build nations as well as destroying them). Supranational institutions never have that as they are superimposed and never command enough loyalty to take a serious common stand against serious adversity.
While people do appeal to the European Court if they think there is a slim chance they'll get the 'right' answer no Briton really expects that the EU is where they should appeal for help if they are being mistreated. Most important rights cases are decided by British courts. When there is a supranational input (such as the ECHR) it is resented and enjoys little popular support. By contrast, despite China having a much larger population and the current regime being even less democratic than the EU Beijing is still where a Chinese person with a grievance will take his or her petition. This has carried on through decades of Communism to the astonishment of outsiders. That means that China is a legitimate authority where the EU is not, even if China is currently run in an undemocratic way.
What that purpose means is that a nation state should be bigger than any one local community but seen as a natural authority to appeal to if your rights are infringed. Of course, that means nations can fail, in light of my definition of their purpose, for two reasons:
1) They become captured by a certain group within the nation who fail to respect the rights of other citizens.
2) They are not seen as a legitimate authority to appeal to if your rights are infringed.
It also means that those who bash nationalism out of a cosmopolitan sense of superiority are playing with fire. If I'm right, and nations are the best guarantors of rights, undermining nationalism risks creating a very dangerous world.