I recall a recent discogs writeup on obis:
https://www.discogs.com/digs/features/the-obi-strip-how-a-japanese-paper-band-became-a-collectors-grail/
Not that informative...conclusion seems to be more or less "just because"...but there is a good quote:
To open a Japanese pressing from the 1970s or 80s and find its obi intact is to encounter a pristine, somewhat mythic art object. Because they weren’t glued down, they were easily lost, damaged, or discarded, particularly by overseas buyers that didn’t read Japanese or didn’t know better. These are records that someone cared enough not to throw away and understood might one day matter. They add a layer of aesthetic completeness, and to some, an almost ceremonial sense of care and preservation.