Show Related Issues allows users to easily see a list of issues that are related to the current issue. The standard way to create relationships between Jira issues is through issue linking. But the relationship between issues is not necessarily through explicit links; it could be through some common attribute such as a common customer, same epic, or all issues part of the same release. This commonality can be expressed as JQL e.g. all the issues with the same reporter as the current issue that are in the Open state.
The purpose of the Show Related Issues app is to give users an easy way to view an issue and all its related issues on the same screen. The app can do this by displaying a panel within the standard Jira issue view, which contains a list of the results of the JQL query that has been defined for that panel. The JQL query finds all other issues that are related by some common set of attributes.