A single Lambda function handling a single task is simple. But what happens when several steps — validation → inventory check → payment → shipping request → notification, as in order processing — are linked together with an order and conditions, and you try to code that flow directly inside Lambda? Each Lambda calls the next one, cancels the previous step on failure, and tracks how far things have completed — and all of that code gets tangled up with the Lambda's business logic. Step Functions pulls this flow control out of the code and defines it as a visual state machine.
This kind of orchestration problem is especially acute in a microservices architecture