If I'm reading the underlying docs correctly, previously you would have called ChangeResourceRecordSets[0] with a quite verbose XML document. It looks like you'd need to first query for the existing RR set, modify it, then update it, and deal with potential race conditions if two service instances are starting concurrently. Technically possible, but quite a bit of complexity.
Now with auto-naming, you create a service[1], then a service instance calls RegisterInstance[2] on start-up with a much simpler JSON payload.