There's a few niches where affiliates provide nontrivial value as well. For example, in travel there are both kinds. A number do take the minimal, often spammy, approach of trying to insert themselves into a sale that was already going to happen so they can capture it. One route is to create content-farm "travel" pages that don't really have much travel information, and exist solely to get an affiliate cookie onto the machine of someone obviously researching travel.

But Hipmunk is another example of a site largely supported by travel-affiliate revenue, and they do it by actually providing a significant value-add service.