Why publish the same command multiple times to reach a position?

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Hi,

I am learning Baxter Robot using ROS in C++ language. For getting used to with ROS and Baxter, I am writing a C++ program to control Joint position. A joint position is defined and it is published to /robot/limb/right/joint_command topic. Below is the snippet of the code-

int main(int argc, char **argv)
{
    //Initializen the ros
    ros::init(argc, argv, "move_joints");

    //Declare the node handle
    ros::NodeHandle node;

    //Decleare a joint state publisher
    ros::Publisher joint_pub = node.advertise<baxter_core_msgs::JointCommand>("/robot/limb/right/joint_command",10);

    //Define the joint state
    baxter_core_msgs::JointCommand joint_cmd;

    //Set the mode
    joint_cmd.mode = baxter_core_msgs::JointCommand::POSITION_MODE;

    //names is a std::vector<std::string>
    joint_cmd.names.push_back("right_e0");//Joint 1
    joint_cmd.names.push_back("right_e1");//Joint 2
    joint_cmd.names.push_back("right_s0");//Joint 3
    joint_cmd.names.push_back("right_s1");//Joint 4
    joint_cmd.names.push_back("right_w0");//Joint 5
    joint_cmd.names.push_back("right_w1");//Joint 6
    joint_cmd.names.push_back("right_w2");//Joint 7

    //position is a std::vector<double>
    joint_cmd.command.push_back(degree_to_radian(30.0));//Joint 1
    joint_cmd.command.push_back(degree_to_radian(0.0));//Joint 2
    joint_cmd.command.push_back(degree_to_radian(0.0));//Joint 3
    joint_cmd.command.push_back(degree_to_radian(0.0));//Joint 4
    joint_cmd.command.push_back(degree_to_radian(0.0));//Joint 5
    joint_cmd.command.push_back(degree_to_radian(0.0));//Joint 6
    joint_cmd.command.push_back(degree_to_radian(0.0));//Joint 7

    //command the robot to move
    joint_pub.publish(joint_cmd);
    ros::spinOnce();

    cout << joint_cmd << endl;

    return 0;
}

The above code DOES NOT work. But when I put the publish method, inside a while (ros::ok()) loop in the following way, it works fine-

ros::Rate loop_rate(10);

while (ros::ok())
{
    //command the robot to move
    joint_pub.publish(joint_cmd);
    ros::spinOnce();
    loop_rate.sleep();

    cout << joint_cmd << endl;
}

I was expecting to move the right arm and reach to the desired position in a single command. But it did not move in a single command. My question is that why we need to send the joint command multiple times? What is the reason behind this? More preciously, how many times a command must be published so that it executes?