[legitster]

Replying a bit late:

This is the difference between "Reach" (number of people who see the ad) and "Impressions" (number of times an ad was shown). Advertisers have the ability to optimize for either (or neither if they optimize for something else).

Usually this is based on some internal metrics they have, like a report that correlates the number of impressions it takes someone to convert. Nobody will buy a product based on a single impression, so some amount of repetition is required. There are also industries with really long sales cycles where consumers may only make buying decisions once a year or less, so if you buy ads, you need to do it over a long period of time.

That's not to say every advertiser is doing things logically or efficiently. There are a lot of dumb strategies. Just like the stock market, it's full of really dumb decisions, and it only seems sensible when you stand back and look at it as a whole.