Every ad that runs on a site serves different goals. The advertiser’s goal is to generate sales; the publisher’s goal is to generate revenue.
Most often overlooked is the goal of the user.
The assumption, unfortunately, is that most every time you run an ad, you’re going to bother the user. There are rare exceptions to this – occasions in which you’ve delivered the right ad to the right person at the right time, the user engages and ultimately clicks through.
That happens only six out of every 10000 times the ad is served – and that’s a generous estimate.