A joined report is a type of report that allows you to create multiple report blocks that provide different views of your data. Each report block can have its own fields, columns, sorting, and filtering. A joined report can only be created from summary or matrix reports. You can use a joined report to show data from different report types or data sources on a single report. In this case, you can use a joined report to show Accounts with Open Cases by creating two report blocks: one for accounts and one for cases with a filter for open status. You can then join the two blocks by a common field such as Account ID or Account Name. References:
https://help.salesforce.com/s/articleView?id=sf.reports_joined_format.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.reports_examples_joined.htm&type=5