By hiding a summary report's Details section, you avoid overwhelming users with data they may not immediately need. When the Details section is hidden, users navigate with the Group tree first, to locate the desired data. Then, by drilling down on the report, they can request specific data, which is returned quickly without unnecessary records.
To facilitate navigation in this way, you first need to group the data and insert the summary fields you want to include in your report.
Hiding details in a summary report