Do your research and ask questions before you agree to join the affiliate partnership. Make sure you have a complete understanding of the vendor’s conditions, payment terms, and what will be expected from you as an affiliate.
To continue our earlier example of a rowing for weight loss website, for example, you may have visitors who are: Just starting to learn about rowing as an exercise form for losing weightShopping for their first rowing machineSeeking advice on rowing exercise routinesShopping for a better/more advanced rowing machineConsidering other aspects of weight-loss to complement their rowing workoutLooking at other complementary exercises to make them better at rowingNeeding to solve injuries related to rowing

And, just like with how-to guides, this format allows you to promote all kinds of affiliate products and services in an organic way: you show people how to do something and then you recommend them the products and/or services they need in order to achieve those results.
Now, also, if somebody clicks through, grabs this book, and then also buys toilet paper and diapers and a new Mac Pro or whatever, you get paid on all that stuff too. Not only do you get paid for this book, but you also get paid for everything else that they buy for the next 24 hours. That’s called your cookie life cycle or whatever.