Gastroliths aka gizzard stones are common in birds, crocodilians & some dinosaurs. They're thought to aid digestion in toothless / non chewing animals. I remember being fascinated by the opalised gastroliths in the fossilised plesiosaur at the Sydney museum when I was a nerdy teenager.
Snakes don't seem to have them. Occasional stones are likely due to accidental ingestion. Question is why don't snakes have gastroliths? They don't chew their food, they're not particularly fussed about carrying a bit of weight. Seem like the perfect candidates. Only answer I can think of is that rodents, birds, lizards are easily digested & stones are not required. Dunno.