In Fashion: When a customer is viewing a pair of pants, the products frequently bought together with it may be shoes or tops which go together with the item, because people are likely to have bought things that match aesthetically.
In Toys: Items frequently bought together with the toy currently being viewed are likely to be those that belong to the same set, or that are popular among those with the same preferences.
In Electronics: A customer viewing a tablet may see frequently bought together recommendations for cases, screen protectors, or other accessories that are popular for that particular tablet.
In Travel: When a customer is viewing a flight the algorithm can show them travel insurance packages and hotels which were frequently bought with that flight.