It's there to make sure that Octane indexes are created only when an explicit create-index request is given to elastic. When coming from Octane, these requests provide the specific mappings and settings required for the index based on it's type (search, trend, event_store, etc.)

So in case the elastic server received an update-index request for a non existing index - it will reject this request.

If you do not  have this setting, and the above situation occurred - elastic server will create an index and then will apply the update-index operation. In this case, the index created will be of bad form, because elastic can't "guess" the specific mappings/settings based on the update-index request.

Having said all that - this happens in rare cases, usually resulting from misusage. For example, an index was deleted directly from the elastic server while Octane is running.