That's true if your school has a large cohort each year that consistently performs around the same level. For example, say last year got around 20 people out of 80 in English (Adv.) that achieved Band 6 - this means that in order to increase your chances of a Band 6 in English (which as such increases your ATAR estimate), you should be in the top 20 in your cohort as this was the amount of people that achieved this mark last year.
However, if you're like me and go to a small school where some years are stronger than others, then this whole notion of 'Band 6s last year' isn't as useful. For example, last year's cohort in my school (2016 cohort) was extremely strong and 7 out of 15 people achieved a Band 6 for English although this number was a lot less (I think) in the 2015 cohort.