I see that most(if not all) of the Magento core modules have a
I could not find any documentation on this, so basically I want to know, what is this file used for ?
This is the file where you define the module access control list resources.
Those resources can be seen under the admin : System > Permissions > User Roles section when you click on Add a new Role button.
If you're not familiar with ACL, I suggest you read this good article from Alan Storm (even though it was written for M1 it is still relevant to understand the functionality): http://alanstorm.com/magento_acl_authentication