I always assumed it was a way to sidestep discussing whether certain contested territories were legitimately "countries" in their own right. By which I mean places like Palestine, Taiwan, and the way North and South Korea each kinda claim the other as part of their territory.

To sound geopolitically sophisticated. To pretend at the language of statecraft.

Because there are lots of synonyms.

But I could have been more specific that nation state tends to imply government of a country. Whereas country might include all citizens.