just a way to show your trafic...... advertisers (some times) check your website's trafic its origin, your alexa rank etc to decide to advertise
oh hmmmm by the way what other statistics you see to decide to advertise on a site (respources to check trafic and its origin)
There are some major advertising networks such Text Link Ads that uses Alexa to determine the price of advertising on the publisher sites/blogs but however not all of the networks do the same.