Purpose of sales.xml File in Magento

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

From what I have found on this one, row_id is the identifier for the row, while entity_id is the actual entity.

There are situations where a Scheduled Update is set to run for a fixed period and in this case a whole new row is created to associate with it's variant eav_attribute(s) and it can retain the same entity_id while being issued a new row_id Then the row_id is changed to the new temporary entity and then back to the original once the schedule completes.

There may be other use cases where they make use of this, this was one which I witnessed and illuminates how it can be used.