Permission Inheritance

SharePoint utilizes permission inheritance so that permissions defined at the top level of a document library, list, or site will automatically be applied to all content under it. 

Permissions should be inherited from the parent site, library or list whenever possible; do not break inheritance unless absolutely necessary as it creates administrative and security overhead/management problems and could negatively impact the performance of the site and searching. It is also recommended to use Active Directory (AD) groups that will automatically update with the members that are added/removed to your organization/departments.