National capitols are perhaps traditional places where a territory or nation could be ruled from. Before the rise of electronic communication or transport machines ruling cities where lived the powerful were accessed by human or horsepower generally. Many cities in drier areas were founded along waterways or confluences of waterways. Farm to market roads and other trading routes culminated at cities of civilization. Plainly it would be possible today to have a nation without a physical capital and it could instead be located on the Internet with a non-profit server not reliant on Google or some worse for-profit governor of free speech.
A physical capital is costly and vulnerable to insurrection and takeover by just a few hundred people with zip ties and legislators must hide in physical basements to avoid capture or summary execution. Internet politicians could instead have the invulnerability of trolls technologically rather than just through policy, of course, so it might not be an improvement.