I can't explain how it works as I don't understand the logistics, my wife's a marketing affiliates manager (I don't know what that means either).
One of her clients is a big supermarket that looked at introducing a mobile phone network to rival Tesco Mobile and she showed me before that you can look at the entire online marketing space and see how much different companies are spending on certain marketing campaigns, for reaching a certain audience, and how much ROI they are making in those campaigns.
I.e. I'm still interested in a festival but the lineup is meh, so asked her to do her magic and see how sales are going, whether they would introduce marketing campaigns to shift more tickets.
Bit of a jumbled sentence, I really don't know how it works.
Basically in the online space, whatever you advertise and market isn't private how much you spend, how much money you make - people like my wife analyse the data and develop marketing campaigns and advise clients to outbid on key words or avoid key words that other companies use.