I'm guessing that you're referring to the need for school ranks to get ATAR estimates here, it isn't actually BOS which provide school ranks but usually it's newspapers eg SMH that calculate them based on the percentage of band six responses at your school.
As you can see, it is a calculation done retrospectively so it doesn't actually affect your marks or ATAR at all. The reason that people ask for it here is because coming 7/90 at say, Sydney Girls is FAR better than coming 7/90 at Vincentia High or another crap school. It would take a lot more talent to get a good rank in a better cohort so people use this information to gague the quality of the cohort...
Hope this helps