Ok, jumping in here. So yes, HN doesn't 'need' a full app, but the functionality difference between this site and Reddit isn't that great. So why, pray tell, does Reddit nag me every time I visit on mobile?

It certainly isn't that the app provides functionality that isn't avaliable through the browser, but somebody decided that it was worth spending development time and nagging users over. So yes, there are things that can't be implemented through the browser and require an app, but it's incorrect to state that nobody develops an app unless they absolutely need to.