Category pages are used when a customer is looking for a particular type of product, but not a specific product such as t-shirts, sofas or running shoes. If they had a specific product in mind, they most likely would have looked for it first using the search function.
Categories tend to have many products, so it can be hard to navigate them but many customers are looking for the same few products in the categories. These select products tend to be the same for each customer, so they will likely be the best-sellers. This is because best-sellers offer the customer a sense of security - if the product was popular amongst others there is some proof of value.
By placing these items at the top of the category page as a group of recommendations, it makes it much easier for your customers to navigate. It is likely that these products will pique new customers’ interests and therefore increase the chances for conversion.