Because you never know what trivial bit of information may ultimately prove to be vitally important.
Ah, one of my most prized possessions is a 1948 Army-Navy hymnal I bought in Groton back in '99. (Well, actually, I lived in Groton but I believe the bookstore was in Mystic.) I bought a similarly-aged Lutheran hymnal at the same place.
Previous post on this topic at http://blogonomicon.blogspot.com/2008/08/collecting-hymnals.html