Composite aggregations in Elasticsearch 6.1 are a significant advancement that enable efficient pagination of large datasets, as demonstrated through an example involving Sliceline, a fictional pizza delivery service. This feature allows for seamless navigation through large result sets, creation of new indices from aggregation results, and consistent API response times. Unlike previous versions, which required retrieving all preceding results, composite aggregations allow users to paginate through results without needing to skip and limit, thus reducing resource consumption and ensuring fast response times. The introduction of composite aggregations facilitates various tasks like building analytical services, creating histograms, and calculating averages, while also being integrated into upcoming Elasticsearch features such as data rollups and SQL group_by. This enhances the ability to handle large datasets efficiently and reliably, making it a valuable tool for data-heavy applications.