SPDT could be used to isolate a section of track for programming purposes. One way, everything works around the layout. Thrown the other side, only the desired segment of track gets the power...or signal in DCC.
DPDT is for reversing sections where you have a loop doubling back on itself, a turntable that does not have a built-in split ring reversing capability, or on turning wyes. Thrown one way, the powered axles can cross a gap into the loop with no problem. Once the entire train of powered axles is across that first gap, and before the first such axle crosses another gap at the far end and returns the way it came (or is reversed in direction), you throw the toggle to the other side of the switch, effectively reversing the polarity so that there is no conflict with the oncoming polarity.
Might sound confusing or complicated, but it is quite straight forward and doable.