Approximately 30% of visitors online, prefer using the search site box for searching any product. It is these users who perhaps show some intention to make a purchase of a possible product.
Using this feature, customers can just insert their query and simply click on the search button to narrow down their result. It is quicker and easier to find a product, especially if your site has too many products to offer. Quicker search-ability means, you are reducing the extra effort that your customer might have had to implement, had there been no such option.