One of the most common use cases is joining data from two different sources. Sample use case:
Data cleansing and preparation
Data scientists at a telco need to prepare data for model building - they need to get the account data from database; CDR and IVR data from S3. They would like an easy way to bring all of the data into a single place, cleanse them and prepare the data for annotation. All the data is keyed by account id.