If the motor gets removed for any reason , that little arm falls down and you don't notice it that easily when refitting things etc .
As has been said before , many have been caught out by it and it is easy to bolt in place again . It helps to stop the motor from moving/ twisting
in the frame which could lead to the chain jumping off .
It caught me out too , i only noticed it as i wheeled the bike out the garage and heard the chain rubbing it - i then knew where that extra bolt went that was left in the tray