Question about finger mimic joints in URDF

[maxymczech]

Can somebody please explain the purpose of mimic joints on finger links in nao_description URDF? Why are fingers set to mimic the hand joint? Example of what I mean (from nao_robot/robot_description/urdf/naoV40_generated_urdf/nao.urdf):

  <joint name="RFinger23" type="continuous">
    <parent link="RFinger22_link"/>
    <child link="RFinger23_link"/>
    <origin rpy="0 0 -0.999899" xyz="0.01436 0 0"/>
    <axis xyz="0 0 1.0"/>
    <mimic joint="RHand" multiplier="0.999899" offset="0"/>
  </joint>

Thank you.