Big audiences produce more chances of your sponsor finding a customer. This is the advertiser goal and you will keep the sponsor if you deliver customers. Big advertisers are looking for numbers. They want to know how many people visit your site. They know that only a small percentage of your audience will click on their banner to visit their site so they need big numbers.
Big numbers have to be measured.
Originally, webmasters could only look at their logs and produce 'hit' counts. This would produce exaggerated numbers. Needless to say, visits were what people wanted to know. Also, you produced those numbers and the advertiser would like an independent company to audit your audience. When a browser reads your page, the logo is retrieved from the audit site.