So if we ignore the technical merits based on which append-only data structures were chosen in general, we don't necessarily have additional technical merits for any particular time append-only was chosen.

The reasons why append-only structures were chosen in general apply surprisingly often to any particular system that scales to large data, which would like to be robust to a wide variety of real world problems. You won't see it in looking at the data structure because the hard parts are abstracted away from you. But you'll see it if you try to reimplement the same system from scratch, then scale it up to production.