What is a Use Case?
A use case describes how an actor (user, system, or time trigger) interacts with a system to achieve a goal — the main success path step by step, plus alternate flows and the exceptions where things go wrong. Where a user story is a conversation placeholder, a use case is the conversation's complete transcript.
They earn their weight in complex, rule-heavy interactions (payments, compliance workflows) where the exceptional paths are the requirement; simple features do fine with stories and acceptance criteria.
Worked example
"Withdraw cash" as a use case: actor — cardholder; main flow — insert card, PIN, amount, dispense, receipt. The value is in the exceptions the story format would have skipped: wrong PIN ×3 (retain card), insufficient funds (offer lower amount), dispenser jam mid-count (reverse transaction, alert branch). Every one of those paths is code, testing, and audit scope — and the use case is where they stopped being surprises.