All of these things—time zones, leap seconds, collations—are gnarly details of the real world around us. And they can, and do, change. Collations matter to PostgreSQL because people want to be able to use natural language strings in database indexes (and use these indexes to drive user-visible ordering.) Hence, PostgreSQL needs to be able to cope with the fact that the underlying collation rules change over time—and that the providers are external libraries we don't control.
Thanks for reading. I hope you found this deep dive into the proposed collation version tracking feature interesting!