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.