Two 24VDC Sources And Diodes
It would work fine without the diodes in an ideal world, where both voltage sources would really have exactly the same voltage. But in the real world this is not the case - both voltages will differ slightly from each other (e.g. one has 23.99V and the other one 24.01V).  

This voltage difference between the sources will result in a compensating current between the sources, which can get quite large (because both supplies are connected via very low resistance), in terms of many amps, or whatever the supplies can source.  

Diodes in between the sources will keep the supplies from "charging" each other. I have no idea, why the diagram shows four diodes, of which two are not used...