When a client is browsing products on your website, they're showing an interest for those at a given time. Thanks to this feature, those products will be shown to the customer as he navigates onto other product pages, which allows him to purchase them easily if the realize that they were what he was looking for, or that the price corresponded more to what they intended to buy.
This feature is really useful when it's associated with a cross-selling feature, and it's possible to combine the two in tabs. By reducing the number of clicks needed to purchase, you're optimizing the sale funnel and improving the conversion rate of your site.
It also allows you to improve the browsing experience for your customers, and in addition to the sales it can generate, it also improves the internal linking of products within your website, which is excellent in terms of SEO : everybody wins with this !