Think the extreme case - what if the site is down for a whole month, a whole day? Then you can understand. Another way: you have nothing else to do at the time

Think the extreme case - what if the site is down for a whole month, a whole day? Then you can understand. Another way: you have nothing else to do at the time. so you might click the ads. But now the site is down, there is no ads for you to click.