Connection strings for Azure IoT Hub begin with "HostName" because this component identifies the specific IoT Hub instance that a device or application will connect to. The "HostName" is part of the structure of the connection string, which usually follows a pattern that includes key elements such as the IoT Hub's name and its associated service endpoint.
When establishing a connection, the connection string must direct the traffic to the correct IoT Hub, and "HostName" effectively serves this purpose by specifying the domain associated with the IoT Hub. This is crucial for ensuring that devices can efficiently communicate with the intended service, as a connection string without the correct hostname would not be able to establish a valid connection.
In contrast, other options such as "Primary Key" and "SharedAccessKey" are components of the connection string that relate to authentication, providing security to the connection but do not serve the primary function of identifying the IoT Hub. The term "azure-iot" is not part of the standard connection string, as it does not reference the connection structure defined by Azure for IoT Hub.