Cross sell is an important aspect of business because it is cheaper to target existing customers than it is to attract new customers.
During an e-commerce experience, when a user is ordering an item through a site they might be offered similar items or items that other users bought when they bought the item being viewed. After any purchase is made, ads will appear for similar style of products which are targeted to the customer due to their propensity to purchase an item through that site.
Increasing the income per sale is one goal of cross selling, as well as maximizing advertising opportunities. Cross selling is only legal if there is no requirement on the buyer to purchase one item in order to be allowed to purchase another item.
Sometimes businesses work together to cross sell each other's product lines in order to share in mutual customer base opportunity.