Substrate Heating

OK, just when I thought I had it figured out, I hear something different.
Is the purpose of heating the substrate:
A: Plants need warm roots

B: we are trying to create alternating zones of warm & cool so that we 
improve water circulation and therefore nutrient exchange.

C: By heating the substrate, we improve the plants' abililty to use the 
available nutrients in some magical way that we will never understand.

D: It gives us something to spend our money on, and is really just a mean 
trick played by those crazy guys at Dupla.

E: None of the above