I guess these advertisers own stores, eg clothing. The marketers also only need to compare between multiple campaigns, not get a fully accurate count of how man

I guess these advertisers own stores, eg clothing. The marketers also only need to compare between multiple campaigns, not get a fully accurate count of how many people were served an ad then made a purchase. Yeah, if you use a Visa card you won't show up, but that's just a fixed unknown factor.