Back heat vs belly heat:

Belly heat allows the snakes to heat up even if the ambient temps in the room are quite low.

In most cases with baby snakes, back heat is just fine. It does not take much to heat a tiny snake. For larger snakes, back heat only works if the room is kept somewhat warm, or warm enough where the back heat can get the back part of the tub up to the required temps. Again, a snake with lots of mass (e.g. ball python) can use back heat only if the room temp is high enough.