I remember seeing a video showing the difference in flame-spread and smoke development between CM (General Purpose), CMR (Riser) and CMP (Plenum) which was sufficient for me to spend a few extra dollars even if the code doesn't require it (I thought it was at the UL website but couldn't find it now).
Here are a few different types of cables and how they react under a flame. Below is also a video of two different alleged plenum cables. The one on the right I think would be an inferior cable if not an outright counterfeit.
With all the combustible material in a typical home a fire will spread regardless what type of cables you use. However plenum cable will at least not actively help in spreading the fire around (as plain PVC will do).
I think that is the main benefit of the plenum rated cables. It's not that it won't burn. Or that it's not toxic if it does. Both of those things can happen. It just helps prevent fire spreading to new parts of the building faster by using air plenums that often have moving air currents that would easily "fan the fire" so to speak. Anything that helps delay the spread of fire is beneficial, and if it releases less smoke, particularly the thick dark smoke that's typical when PVC burns, all the better.