To get free/busy information from users calendars (https://docs.microsoft.com/en-us/graph/api/calendar-getschedule?view=graph-rest-1.0&tabs=http), the API permissions Calendars.Read or Calendars.Read...
Many O365 customers wish to provide access to calendars to 3rd parties but want to restrict specific actions such as access to attachments and meeting notes. a ".basic" permission is read only but 3rd parties need readwrite but with more granular restrictions.