Land line telegraph keys are fitted with a circuit closer switch. Radio telegraph keys were not but many radio operators used land line keys because they were so readily available. This includes the rather ubiquitous J-38 about which more will be found below. The arcing that occurred at the key contacts of the early spark radios caused radio stations to switch to purpose made keys which had more robust contacts and in some cases arc shields. In North American practice the idle condition of a land telegraph line was with a current of ~60 - 90 Milliamperes flowing continuously and the ferrous metal armatures of all Relays, Main Line Sounders, and Registers, were used, pulled to their energized position by the magnetic flux of the Electromagnet which is part of all Land Line telegraph receiving apparatus. As others have indicated the receiving apparatus were in series in the lines which passed through each station. When the line was idle the continuous current flowing through the line had to pass through each set of Relay or Main Line Sounder electromagnet coils in series.
The earliest batteries used in land line telegraphy were Gravity Cells, so called because the only thing separating the two liquids that made up the batteries electrolyte were the difference in their specific gravity. In order for those Gravity Cells to remain functional they had to be making current continuously. If the circuit was opened for any long period of time the specific gravity of the 2 liquids would become too similar and they would then mix rendering the battery useless until the output was reestablished and the two liquids would separate after the passage of sufficient time. That was the largest factor in telegraph lines in the North America being normally closed constant current systems because they started out that way.
If any telegrapher left their circuit closer open while they were not transmitting it would cripple the line. Without all other circuit closers being closed no operator could take control of the current flow to transmit a message. By opening the circuit closer switch the telegrapher took control of the line and caused all of the other sounders on the line to de-energize. The absence of current flow through the electromagnets of all of the Relays and Main Line Sounders, connected in series in the line, would result in the loss of the magnetic flux generated by each set of coils thus releasing the armature of the Relays and Main Line Sounders to be pushed, in the case of sounders, or pulled in the case of ralays, to their relaxed condition by their return springs. The single click made by the Sounders when the return spring pushed the hammer bar up against the limit stop of their frame alerted the other stations' operators to listen for their station's call to see if the message was for them. If they heard their stations call they would open their keys circuit closer and the operator who was sending their call would know that because the sending operators sounder would go silent in the up posotion. The telegrapher who called the station would then close their circuit closer to allow the other operator to respond. Occasionally the operator who opened the line would be someone other than the station called. This might be done by a telegrapher with urgent traffic such as a railway dispatcher controlling the movement of trains or, especially during World War Two, an operator who needed to send an Army Flash message.
The J-38 Telegraph Key is a US Army Signal Corps LAND LINE key that was already in use when the United States entered World War Two. Every permanent Army post had a telegraph office to send and receive messages which could not wait for the US Mail. The fact that it was a line key can be deduced from the presence of a shunt bar for the other side of the line and the markings on the base plate that indicate that the LINE is to be connected to one shunt terminal and one key terminal and the Relay or Main Line Sounder was to be connected to the shunt terminal and key terminal on the other side of the base plate labeled TEL for telegraph equipment.
Since the J-38 was already designed, prototyped, tested, tooled, and in production at the outbreak of WW2 it was quicker to use the J-38 for many applications than it would have been to wait for purpose made telegraph keys for individual uses to become available. As a consequence many thousands of J-38 keys were made during the early years of the war until the keys which were made for those other uses became available. When those other keys, such as the J-43 et al, became available their were already thousands of J-38 keys in the supply chain which became surplus after the war. That is why the J-38 is by far the most commonly available military surplus key.