joint_trajectory_action question

edit

I am reading the joint_trajectory_action.cpp file in joint_trajectory_action ros package.

I have a question about following callback function.

void goalCB(GoalHandle gh)
{
  // Ensures that the joints in the goal match the joints we are commanding.
  if (!setsEqual(joint_names_, gh.getGoal()->trajectory.joint_names))
  {
    ROS_ERROR("Joints on incoming goal don't match our joints");
    gh.setRejected();
    return;
  }

  // Cancels the currently active goal.
  if (has_active_goal_)
  {
    // Stops the controller.
    trajectory_msgs::JointTrajectory empty;
    empty.joint_names = joint_names_;
    pub_controller_command_.publish(empty);

    // Marks the current goal as canceled.
    active_goal_.setCanceled();
    has_active_goal_ = false;
  }

  gh.setAccepted();
  active_goal_ = gh;
  has_active_goal_ = true;

  // Sends the trajectory along to the controller
  current_traj_ = active_goal_.getGoal()->trajectory;
  pub_controller_command_.publish(current_traj_);
}

I don't understand why it needs to cancel the currently active goal.

Thanks for any help in advance!