Why do networking people use the term “BGP Speaker” instead of “BGP Router” ? The simple reason is that in many networks there are other devices that do not route traffic at all, but exist to gather information on the BGP data. For example, Route Servers that exist to only act as Route Reflectors for all routers in the network do not pass customer data at all. That is, their data plane is unused except to transport BGP packets into the network.
Note for other routing protocols we use the terminology “OSPF Router” or “RIP Router” quite freely and it’s only BGP that has this “speaker” concept.