What is the use of the merger class in Magento Message queue?

Question

Can someone kindly explain the use of the Merger class when using Magento Message queue framework? Also, why is there no default merger class ?

Answer 1

I had to read up myself and the concept also confused me

I found the following online and the context of what's trying to be done this makes sense.

This [merger] class can be used for preliminary processing of messages even before the class that was declared in queue.xml in the Consumer section. It is obligatory to use this class, but if you do not require preliminary processing of messages, then you may simply return messages back

https://belvg.com/blog/how-to-use-rabbitmq-queue-in-extensions-for-magento-2-0-ee.html

Not sure what you mean by no fallback class - sorry.