When to use the `retain' feature for mosquitto_publish

As we know, the MQTT system is consist of broker, subscriber and publisher.
The requirement for my task is when the device boot up, ask the configuration
from the server, this means, I should set my device as publisher, the remote
server is a subscriber.

If the subscriber has been connected to broker, once the publisher connected
to broker, we can publish message successfully.

However, if the subscriber is not connected to broker, though the publisher
connected to the broker, but the we can not publish message successfully.
More worse, the message can be published only one time when the process start,
if the message cannot be sent to the subscriber, it will lost!

check the mosquitto_publish API info.

libmosq_EXPORT int mosquitto_publish(struct mosquitto* mosq,
                int* mid,
                const char* topic,
                int payloadlen,
                const void* payload,
                int qos,
                bool retain)
              

And I notice the parameter retain, meaning set to true to make the message
retained.

Once set the it as true, the key published message will be retained in
broker, when the subscriber connected to the broker, broker will send the
retained message to subscriber, then my issue fixed.

Note:
For the repeated message or not important message, we should set the retain
as false to reduce the broker load.