Remember how, when you buy something on Amazon they are always quick to tell you that people that bought that particular gadget, book, DVD or special gift, also bought these other vaguely (sometimes curiously remotely) similar items?
It's called the upsell. Whilst I've got you buying one thing, I'll sell you another.
I was reminded of this when looking at Custom Affinity Audiences which we are actively encouraging our clients to consider in their accounts. We're seeing some strong results, and the reason seems to be quite simple.