This built-in role has been designed to give a user some broad powers within a project s/he is part of, much the same as a project manager in life normally enjoys in relation to the project s/he manages. The role enables such a user (project manager, for ease of reference) to add projects, add, view, or edit activities, view and edit time logs of her own and other project members’, view and manage project members and assignees, etc.
"Nominal" vs. "actual" project manager
It is important to note that the built-in 'Project Manager' role is only a representative of a group of roles that set aside a user from other users in terms of level of access within the projects they are part of. Project managers, in general and rightly so, have the highest level of access to their projects. It goes without saying. Any role consists of permissions. And the permission that makes project manager distinct from lower access project members is 'Manage members'. Accordingly, if you modify a member's role so that it has this permission enabled, the member, de facto, becomes a project manager, too. Easy Project treats such roles as equal to project manager's. Accordingly, if you want to filter out projects by project managers, the resulting list will include members who nominally may not be project managers, but who are by virtue of their treat-as-project-manager role.