It makes sense to put all the relevent material together rather than get the congregation to bring 4 or 5 books to the meeting. It also means they can rewrite material so it is in line with new doctrine rather than risk people bringing old books to the meeting with old light.
It also means that the faithful are less inclined to go off on a tangent and do their own research.
The expression workbook also gives the impression of something compulsory that must be completed the underlying message is "we have made it easier for you to find the material you need, there is no excuse for not completing the asigned tasks."
They could have called it a guide or study aid but they have called it a work book. That says everything you need to know about this religion.