Why is it necessary to declare ros transform broadcaster in main?

edit

Hi, I was trying to broadcast a tf in ros.

#include <ros/ros.h>
#include <tf/transform_broadcaster.h>
#include <tf/tf.h>
#include <geometry_msgs/TransformStamped.h> 

ros::Timer robot_pose_broadcaster;

void robotPoseCallBack(const ros::TimerEvent& eventalpha)
{
    tf::TransformBroadcaster robotPoseBroadcaster;
    geometry_msgs::TransformStamped robotPose;

    robotPose.header.stamp = ros::Time::now();
    robotPose.header.frame_id = "/map";
    robotPose.child_frame_id = "/base_link";

    robotPose.transform.translation.x = 0.0;
    robotPose.transform.translation.y = 0.0;
    robotPose.transform.translation.z = 0.0;
    robotPose.transform.rotation.x = 0.0;
    robotPose.transform.rotation.y = 0.0;
    robotPose.transform.rotation.z = 0.0;
    robotPose.transform.rotation.w = 0.9999;
    robotPoseBroadcaster.sendTransform(robotPose);
    ROS_INFO("DONE POSE BROADCASTING");
}

int main(int argc, char** argv)
{
        ros::init(argc, argv, "od_transform");
        ros::NodeHandle n;
        robot_pose_broadcaster = n.createTimer(ros::Duration(.1), robotPoseCallBack);

//      tf::TransformBroadcaster aphabetagamma;

        ros::spin();
        return 0;
}

This code broadcast nothing. But when I uncomment the below line.

tf::TransformBroadcaster aphabetagamma;

This code publishes transform between map and base_link even though I never use variable aphabetagamma. Can someone help me understand this.

Thanks.