A message broker allows for asynchronous and decoupled data transfer. With a protocols like HTTP, an unresponsive node could slow down other nodes. With a message broker, messages to an unreachable node may be delivered as a later time.
See this list of client and developer tools that support
Nodes MAY have an HTTP REST API which can be used to interface with the event chain service. Other node SHOULD NOT use the REST API. It's only intended for client applications hosted by the owner of the node.