I pretty much agree with what Grant VG said.
We use many different sources of heat, we use some heat tape where it applies.
Grant touched on ambient temperatures and humidity. From my experience, that is the key to keeping boas and pythons happy.
I have a feeling, that many people who use heat tape have too low an ambient air temperature. You know, 75 degrees ambient temp, 95 degree hot spot. The air the python breaths should be warmer, and I use a variety of under heat, side heat, lights and radiant sources - as well as a higher room ambient to create that.
There are many ways that work, do what works for you.
So your question "why belly heat is necessary"