You would like to know why are Aurea Messenger (Sonic) brokers are required for Aurea Monitor (Actional), because while upgrading your Aurea Monitor (aka Cx Monitor / Actional) to version 12.0 (2021) you were asked to introduce two new message brokers in Aurea Messenger (aka Sonic), eg: brMonitor1 and brMonitor1Bak, in order for Aurea Monitor to function properly.
With the release of version 11.x of Aurea Monitor a new architecture got introduced.
The communication between the Agent and the Management Server previously was a synchronous communication done on HTTP.
With the architecture change, the communication was changed to asynchronous, and instead of HTTP, the architecture was changed to use JMS.
Aurea Messenger is one of the supported JMS Buses that can be used for Aurea Monitor Agent - Server communication.
The complete list of supported JMS buses for Agent - Server Communication of Aurea Monitor is listed in the Supported Platforms Guide.