So when purchasing from Tier 1 sellers why does it have the option to check it or not? Aren't they all Tier 1 sellers? ????
No, all sellers are not Tier-1 sellers, i.e. not all of their traffic from tier-1 countries.
So, checking the "Only Top Tier" option ensures that only the clicks which are delivered from the top countries are counted.
Hope this is helpful :)