SharePoint 2013 Permission Inheritance Brief
When it comes to permissions on securable objects in a site, they are inherited from the parent object by default. That inheritance can be broken and then you can use fine grained permissions. This is a way to have more control over the actions that users can make on your site. Without inheriting permissions, all of the permissions are explicit and that means changes to the parent don’t affect the child object.
If you were to restore inherited permissions, then the child object will also inherit its users and groups. You will then lose any groupers or users that were given unique permission levels that came from the child object. With that in mind, you should always rely on permission inheritance when you can.
If you wish to break inheritance so that you can use fine grained permissions, make sure you use groups so that you don’t have to track individual users. People tend to move in and out of teams and that results in their responsibilities changing often. It can consume a great deal of time to track those changes and to update the permissions for uniquely secured objects. It can also increase the risk of errors.