Stops your nozzle getting pushed into your platform if you go below z0 for any reason.
Make the height of this 1 to 2mm less than where your limit switch is set. The STL is 50.5mm from where it sits in the platform to the top. Scale the STL in Z to adjust for height. The underneath part won't change much with this scaling so don't worry about it. Alternatively you can adjust the Sketchup file and output a new STL.
I've been using this for a few wekks now and it has saved me once.