It was the place where OLE controls "registered" themselves, to announce that they were available to all potential users. The binary aspect of it may have been the optimization for really slow and small machines of that time (think Win 3.1).

Then came the push to store everything in the Registry. And the pain started.

Any advanced environment needs a place where common things can be found. Gnome is reinventing it. I am pretty sure you can find it in KDE too, although I don't know for sure.

Acrobat does not need to announce its own availabilty to anything. At most, when you started it, it had to reestablish file extension associations. But that is done anyway, nowadays, because people steal them left and right.