Please do not take the word of people whose only real expertise lies in posting opinions.
Please do not take my opinion.
Read the hundreds of web sites available to you through a simple internet search.
I think:
Vitamin D, located in certain blood cells in the skin is converted by part of the UVB light band to the hormone known as Vitamin D3. This is converted in the liver and transported to the kidneys which excrete the active form used in the cells of the intestine to metabolise calcium.
Without this process ingested calcium is not metabolised properly and the animal suffers from Metabolic Bone Disease.
Feeding extra calcium alone will not help, because the calcium can not be used without the Vitamin D3 hormone. In fact, an Old Wives Tale suggests that a way to kill mice and rats is to get them to walk through calcium powder so that when they lick it from their paws it settles in the gut and ultimately kills them. (I'm not saying I believe Old Wives Tales, just that feeding a lot of indigestible calcium may not be such a great idea.).
In reptiles, hatchlings and juveniles are particularly susceptible to MBD because of their rapid growth. Conversely in humans, the problem becomes evident with age because the skin cells cease to function properly. Hence older humans are encouraged to take Vitamin D3 supplements.
However, an overdose of Vitamin D3 is highly toxic and there is no real agreement among doctors on safe dosages for humans, let alone reptiles. Feeding "Vitamin D3" to reptiles would be a waste of money if it were the inactive cheap form, and it would be dabbling in a dangerous poison if it were the actual steroid needed.
Dragons, because of their habitat, are forced to spend long periods exposed to deadly UV rays. They have developed skin capable of protecting them. Presumeably, very little of the UV light striking them gets through to the cells that use it and so they not only can stay out in the sun for hours, but it may be that they actually have to.
Geckos on the other hand seem capable of collecting enough UV light in the cells of their thin skin in a very short time at dusk or dawn.
Vitamin D3 is stored in body fat and present in the blood, skin and major organs where carnivores like dogs and cats, and presumeably snakes, can access it. So it may be true that snakes do not need UVB light. Alternatively, in snakes, the skin cells that use UV light may be located so that they need very little exposure, like a gecko?
Vitamin D3 remains stored in the body for a very long time and exposure to UV light may not be needed very frequently.
To say a snake does not need UVB light is making a dangerous assumption. In the wild, pythons emerge at dusk and like geckos, they absorb UVB rays. They come out in the day time, in shade, but UVB light is known to penetrate dense forest.
It may well be that a short time in the shade allows a snake to use UVB to activate enough Vitamin D3 to last it for months. Who knows? Please make up your own mind by reading the research available on the internet - it's not all scientific gobbledegook or disguised advertising.