In the most basic sense, it’s a “sales person” representing another business. It’s an ‘affiliation’ with another seller.
For example, if your business was selling football shirts and shorts, it would make sense to become ‘affiliated’ to a company selling footballs. If you ran a photoshop, it would make sense to join up with a company selling camera items.
In internet marketing, this theory is sort of ‘thrown out the window’ If they see a new product being sold, they want to get a share of the pie, so they will sign up as an affiliate. Last week, they were selling computer apps to their readers, this week, they are promoting a horse racing system. Then they wonder why they get very few sales.
So if you’re selling to your previous buyers, it makes sense to offer them more of what they have purchased in the past.
“What about (platform X/Y/Z) ? They have a product directory that promotes anything and everything. They don’t specialise” Well, in a sense, they do. They put the products into categories. So anyone interested in gambling systems will look for products in the betting section.
It comes down to “knowing your buyer”, and not offering them items they have never shown an interest in before.