With this they go one step ahead to optimize and minimize the number of queries, unifying as many small queries as possible into one and gaining performance in the reports.
A great performance improvement (Horizontal Fusion) merges multiple smaller data source queries into one larger data source query.
It is a big query performance optimization in PowerBI and Analysis_Services when processing DAX queries, by reducing the number of data source queries needed to produce results.
This decreases the number of processing requests on large data sources and therefore reduces the response time of DAX queries, resulting in a very significant performance gain.