Small devices, local computing, visualization services, mobile applications: when these nodes need to communicate each other using the Internet they can't be put in direct connection. This is caused by several factors.
End points are partners of communication (the flow of messages) in the form of devices, products, servers, computers designed or prepared by the customer.
A messaging protocol paradigm is the only choice in many situations. It means that communication between two or more partners is possible because a server in the middle accepts and route messages at the low level. Think of a chat room for devices.