The B vitamin shots are definitely more common for basking species like red ear sliders or bearded dragons from what I've seen. For the sliders, a common issue is the watery or swollen looking eyes and that is sometimes due to a B vit deficiency.
They work as an all around immune booster from what I understand. In this case, a B-12 shot is to help get the intestines moving a bit more. And the worst that will happen with a single shot would be the snake just pees it out later. *Shrug*