Purchase with purchase means to offer a discount on a second product on the condition that the customer buys the first product. It is primarily done with the purpose the sales of the primary product but can also push the sales of the secondary product.
Most of the times both the primary and the secondary product belong to the same company and many a times the same brand, but a collaboration between companies is possible if they are offering complimentary products. A point to not that in purchase with purchase offers, you can’t get the secondary product for free but you get it at a discounted rate. It is always not necessary that you have a secondary product offered at a discounted rate in a purchase with purchase offer. It can be that both the products are offered at a discounted price if bought together.
Examples – On buying a PS3, you are offered PS3 games at a discounted price; on buying a snack you can be offered another snack or beverage at a discount; a single tee can be sold at say 499 INR and you can buy two tees of the same model at 399 each.