At least I know what it is doing at the moment its blocked (and in many cases I can tell whether that is justified to take N time or not).

With a progress bar I don't know anything.

I don't know whether it _will_ be any different, but there was some speculation about that:

> If the users can see the detailed tasks, maybe they will be less ticked off by a jumpy progress bar.

Context. You see not just the thing currently executing, but also the things remaining to be executed - which allows you to mentally gauge which of them may slow down.