Every project is executed by different people, each with their own set of responsibilities. Usually, certain aspects of the project are taken care of by people who are part of a team. For instance, in the case of a software project, there will be a project lead who manages everything in the project and is responsible for overall progress, developers who work on the features, and then there is a team of testers to verify the built features. The first versions of JIRA had only groups, and JIRA administrators would end up creating multiple groups for various projects, such as proj-administrators, proj-developers, and so on. At the same time, the permission scheme for each project also needed to be defined for these groups. JIRA developers realized this problem and introduced the concept of a project role that allows not only JIRA administrators but also the project administrator to add/remove users to their project roles.
In JIRA, you could have roles that are defined...