I had never heard of collecting napkins, but it seems there is someone somewhere collecting almost every type of thing. It makes sense that if it was a "thing" in your area to do so, then why not?
I can't remember who it was, but it seems I read where someone was responsible for going through their deceased Mother's or Aunt's things and one of those things turned out to be a napkin collection.
Lots of fun designs you have there.