The Product Set Campaigns feature is useful when you want to create a set of specific products for a landing page, front page, etc.
There is a difference between creating a normal Merchandising Center campaign and a Product Set Campaigns campaign.
In the normal Merchandising Center setup, you can choose which products you want to boost and the rest of the products within the module, you have no direct control over since it is based on the individual’s preferences.
In the Product Set Campaigns, you choose all the products and the module will only show these specific products. The chosen products will still be based on relevance to the individual, but only within the specific products that you have chosen.