The information that a person can see and change is usually based on their job title or description for that project. Its faster and easier to give permissions to roles instead of users because:
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Users with different roles can have some of the same permissions in a project. You can give permissions to several roles faster than you can give the same permissions to lots of users. And if the person assigned to a role changes, all you have to do is change their role instead of resetting all of their permissions.
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One user might have different roles in different projects, but permissions for roles are often very similar across projects - You can export the permissions setup from one project and import it into another project. It's easier to make a few adjustments to specific roles in each project than it is to adjust the permissions for every user in each project.
For more help, see Defining Who Can Do What.