Who says that? I said the exact opposite. After you sold something you create a project basically to track the progress.
The invoice was created because of the sale order and the customer now have questions about the invoice. So it would be handy to have the sale order at hand to see what was ordered and what was invoiced. You can enter the sale order number into the event description, but then another employee have to search for it again, which it not very efficient.
From your quote, it seems the CASE is not linked to anything. So it can be used everywhere like in production where something goes wrong. A case is created and different events happen to fix the problem. Then the case is closed.
Another one: Building inspection, during inspection different cases are created and later events happen to fix the cases.
So I think that this model is way more then only a party communication. It can be used everywhere in the system and as a base for other modules. Maybe make it more general (rename) and have it as a first citizen in the system like notes and attachments?