[joefkelley]

I think bidding tends to just be about what the advertiser cares most about. If they are a "brand" advertiser (Coca-Cola is the canonical example) they might bid per impression since they're not really looking for clicks, and they have some idea of how much an impression is worth to them. Compared to say some app where they want installs and so might bid per conversion.

As for the network, it's also not as clear what's profitable. Bidding per click or conversion probably gives them more opportunity to do well with good targeting. But high per-impression bids are also useful for the users the network knows very little about.