I actually meant trace ID and parent event ID (and ID was inferred). Parent comment is correct in that trace ID isnt technically needed, and is in fact quite controversial. Its an implementation level protocol optimization though, and unfortunately not an objective one. It creates an arbitrary grouping of these annotations - which is entirely subjective, and the spec struggles to reconcile - but its primarily because the technology to aggregate and/or query them would be far more difficult if you didn't keep that simple GUID.

It does have one positive benefit beyond that. If you lose data, or have disparate systems, its pretty easy to keep the Trace ID intact and still have better instrumentation than otherwise.