Segmenting by purchase history means you group your subscribers based on what they have already bought from you or your partners in the past. For affiliate work, this is great because you only send offers for things that naturally fit with what they already like or own. If someone bought a beginner's camera, you would not send them an email about advanced lighting equipment just yet; you would send them an offer for a good camera bag instead. This makes your emails feel very personal and relevant, so people are much more likely to click on the affiliate link because it is a good next step for them. Does knowing what people bought before help you suggest better follow-up items?