This is only an opinion but I believe the gradient is more due to their physiology. They need these gradients to thermoregulate just as humans do internally. If you give them just a single temp they are stuck regardless of what internal temp is better for a certain situation, by giving them a gradient you allow them to choose what they need. Depending how far they are below ground they can definitely find temperature gradients below ground. I assume the original idea was to have the mean temp of a tank close to what they would encounter in the wild and have a few degrees above and below to allow for thermoregulation. At least that is what I would do. I definitely think people have tinkered with it over the years especially when you talk about breeding drops and such.