[ashmud]

For processing data types without quality natural unique keys, being able to generate a unique key beforehand is very useful. I've written code both ways: inserting to get an auto-incremented integer, and generating a UUID, and the UUID approach was much easier to write and maintain.

[jameshart]

I'm going to make a bold claim: no datatype has a quality natural unique key.

I am struggling to think of a counterexample. Maybe chemical elements? Where the natural key is of course the atomic number, not the chemical symbol...

Before you suggest 'zipcodes' or 'states', consider that zipcodes are not in any sense natural, and anyway, like state codes, are so US-centric that they don't belong as top level elements in most real database schemas.