Any eCommerce website is going to need to have lots of professional images on it, but these are tough to get. Either you're going to have to hire a graphic designer and/or a photographer to create them for you, or you're going to have to have the skills to do it yourself, either way, professional images are valuable assets and something for you to be protective of - you don't want anybody to steal your images and use them for their own purposes (especially competitors!) You need to make sure you protect every aspect of your business.
This is why you need to watermark your images. If you add a little watermark with your name or your business's name on it, you're not too likely to have your images stolen. It's true that some people might just crop a watermark out, but if you position it so that the image will look sloppy with that cropped out, you should be okay, because there would be not point in stealing it if that were the case. If you don't have the capacity to add watermarks of your own, you don't need to worry too much, because you can find free watermarking tools from Water Marquee and lots of other sites like that.
So I hope that gives you a good idea of how to protect some of your digital assets. If you paid good money for some photography work or for a graphic designer to create something for you, it would be pretty frustrating if somebody then came along and just took it from your site! This can help to stop that from happening and it also helps to make you look a little more professional: the majority of professional sites will be watermarking their images and if you do it too, it will help to make your customers trust you.