Not a super big mystery, but most SCI1.1 games contain a Conversation class that appears to let you queue up a bunch of messages to be displayed in sequence. However, the Messager class already has this functionality, so it seems a bit superfluous (except it would allow you to queue up messages that are from different tuples)
In SQ5 and LSL6, at least, it is unused. KQ6 uses it extensively (but again, it looks like the Message class would serve just as well).
Hmm, actually now that I think about it, it would let you split a sequence of messages into multiple parts, and execute code between each (whereas if you used the MEssager functionality, the messages would be displayed in sequence without any chance to intervene).