HTTP is a synchronous protocol. A client sends a request and waits for a response. This works well for operations that complete in milliseconds — a database read, a record update, a validation check. It breaks down for operations that take seconds, minutes, or longer: video transcoding, report generation, bulk data exports, machine learning inference on large inputs, email campaigns to millions of recipients.

Holding an HTTP connection open for a minute or more is technically possible and practically wrong. Clients time out. Load balancers have maximum request durations. Users cannot tell the difference between a server still working and a server that silently failed. Long synchronous operations produce brittle integrations and poor user experiences.