Ecommerce platforms are mostly used by SMEs which are going for online sales for the first time. They aim at providing a shared solution (made for many webshops at a time, and hosted on the company’s servers or self-hosted - for more information about hosting for ecommerce platform follow this link) to these companies. This is what is called an ASP solution.
All platforms work under the same principle, you can create your webshop yourself in a few steps, and personalize the graphic design using the templates provided, or use a marketing agency to get your own tailor-made graphic charter. All merchants from a same platform have the same administration back-end which enables them to bring their project alive (product addition, inventory management etc.).