A sample is a real, physical example of the product that your customer will receive. So, you can examine and touch it, check the color saturation, test the material durability and everything else related to the quality of the product.
You will be able to check out the potential brand experience that your customers will have. Stickers, thank you notes and other goodies that will make a product uniquely yours.
Packaging is important for both shipping safety and the brand experience. Sample will let you test how safe and aesthetic the packaging really is.