Non-derailing Diodes?

I’m looking for a bit of education so that I can properly wire my switches for non-derailing operation.  The following are 3 diagrams all from Z-stuff.  The first one is a diagram suggesting how to wire for non-derailing and it includes diodes in the circuit.  That is how I presently have my switch wired to both an AIU and the DZ-2502, but not yet for non-derailing.  So my question is why diodes are required?  In the second diagram (shown as Figure 4 within the diagram), there are no diodes when being used with a relay instead of the AIU.  But then in the third diagram also using a relay, the diodes are once again inserted.   Protection for the switch machine in the event of a short?  I continue to learn from the forum and so thanks in advance for your assistance.