Why are state elements used in sdf ?

edit

 <state world_name='default'>
      <sim_time>3535 973000000</sim_time>
      <real_time>232 539712353</real_time>
      <wall_time>1442941265 684361676</wall_time>
      <model name='Construction Barrel_3'>
        <pose>-6.4015 6.0113 10.049999 3e-06 -0 -0.120405</pose>
        <link name='link'>
          <pose>-6.4015 6.0113 0.049999 3e-06 -0 -0.120405</pose>
          <velocity>0 0 0 0 -0 0</velocity>
          <acceleration>0 0 0 0 -0 0</acceleration>
          <wrench>0 0 0 0 -0 0</wrench>
        </link>
      </model>
</state>

please explain above lines of code