Nationalism aims to unite people as members of a nation, and for that purpose, the belief that they really have common national characteristics is obviously useful, even if those common characteristics cannot be proved beyond a shared language and a similar physical appearance.
Reference is sometimes made to the "national psyche" or the "soul" of a nation, to explain why some public events can trigger a commotion or uproar in a country, or why a particular nation gets particularly enthusiastic or obsessed with a sport or cultural practice.