When I first started using Postgres, I sat and thought forever about which PKs to use, and looking back, I was way overthinking it. Combination of unique fields

When I first started using Postgres, I sat and thought forever about which PKs to use, and looking back, I was way overthinking it. Combination of unique fields, UUIDs, hashed data... Now I always use bigserial without thinking about it. When my DBA hat is on, it's none of my concern how the user-facing IDs will look; I just know it's gonna be a string of some kind. What's the common use case for the others? I can imagine for weird performance reasons you might want to pick special PKs, but that implies you're exposing them to clients, which you almost never want. The only more reasonable thing I can think of is a UUIDv4 for a special sharded database.