robot_state_publisher, for gazebo only or for actual robot?

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URDF is not only for Gazebo, or just for simulation purposes. URDF defines your robot's physical dimensions, joints, constraints etc. Therefore robot_state_publisher publishes states of your link in your robot, not necessarily in simulation. For example, also for actual robot, it publishes tf data between your base_link and laser sensor link (like base_laser_link) using distances defined in URDF. You do not need to publish them separately by writing some nodes.

For joint states, if you can publish the states of your joints to joint_states topic, it can also publish those, not only static link distances.