Many backend systems will generate a key in response to a newly POSTed entity.

If a client sends a POST request to create an entity, and the OData service does create the entity, but the HTTP response to the client is lost, the client may be unsure as to whether or not the request succeeded. If the client repeats the request, it may result in duplicate information in the backend system.

Idempotence is especially important for clients which want to be able to do store-and-forward (e.g. offline enablement).