The mirror side of this is when almost all the intern applications/cover letters we used to receive contained a paragraph about the hobbies of the applicant (all domain-irrelevant). I find that weird but I guess it is sort of common nowadays?

I was involved with picking a candidate for an internship in 1999, we had three candidates and they all mentioned basketball on their resumes. I think it's just something to help the resume fill one page. And help provide a direction for a 'validate the resume' interview.

Sometimes, if the hobby shows leadership etc, it might be relevant even if the domain isn't.