It became a street food staple largely because it's one of the few fried snacks considered acceptable during Hindu fasting days (upvas), when grains and lentils are off-limits but tapioca and potato are not.
Soaked tapioca pearls are mixed with mashed potato, roasted peanuts, and green chili, shaped into small patties, and deep-fried until the outside turns crisp while the pearls inside stay chewy and translucent. Maharashtra's street stalls sell it hot with a side of yogurt and peanut chutney, especially around fasting festivals.