What does the usage data access permission allow I only just now became aware of the "Apps with usage access" preference view. However I'm irritated.

The permission was deemed important enough as to warrant a separate setting view to provide control and summary to the user, while at the same time there is almost no explanation as to what it does. On my OnePlus, there is not even a help page. 

So, exactly what kind of access does an app get if it requests (and receives) usage data access? And if usage data I would really like to know what kind of usage data.

Android version is 8.1, should that make a difference.
Not a developer here

Apps granted this permission have access to data pertaining to usage of other apps, like in my case Better Battery Statistics, Macrodroid (automation app) and  such apps that need to know this

App usage statistics 


  You can now access app usage history on an Android device with the new android.app.usage API
  
  The system collects the usage data on a per-app basis, aggregating the data over daily, weekly, monthly, and yearly intervals. The maximum duration that the system keeps this data is as follows:
  
  
  Daily data: 7 days
  Weekly data: 4 weeks
  Monthly data: 6 months
  Yearly data: 2 years
  
  
  For each app, the system records the following data:
  
  
  The last time the app was used
  The total length of time the app was in the foreground for that time interval (by day, week, month, or year)
  Timestamp capturing when a component (identified by a package and activity name) moved to the foreground or background during a day
  Timestamp capturing when a device configuration changed (such as when the device orientation changed because of rotation)