But it isn't. A UUID is a disjoint-union of several different types of IDs, only one of which is just random bits, and if random bits is all you care about, then "random 128-bit number" is not only clearer but gives more entropy than "version 4 UUID".
That what I'm trying to ask, what is the purpose of this disjoint union, when would you ever use the "UUID-ness" of UUIDs (which is not the same as asking about the virtues of UUIDs of a particular version).
The virtue is, software has some idea what a UUID is. There's a mostly-random version which is also understood.
If you want to start doing your own thing, a random number is good. It's hard to get a good random number. I suggest starting with, hey you guessed it, the UUID library.
That what I'm trying to ask, what is the purpose of this disjoint union, when would you ever use the "UUID-ness" of UUIDs (which is not the same as asking about the virtues of UUIDs of a particular version).