A controlled input has its value driven by React state, so React is the single source of truth. An uncontrolled input manages its own state in the DOM, and you read its value with a ref when needed.

For almost everything: forms that need validation, inputs that affect other UI, search boxes that filter live, and any case where you need the value as the user types. Controlled keeps React as the single source of truth.

For simple one-time reads, like a single submit form where you only need the value on submit, or for integrating with non-React code that needs direct DOM access. Uncontrolled is simpler and less code for these cases.