Reseller Hosting allows you to create sub-packages within the allotted Disk Space and Bandwidth of your main Hosting package. You can use WHM to create Individual Custom packages (each with cPanel) and provision them to your Customers. Additionally, you can use WHMCS for billing.
If you're a Web Designer/Developer you can host and manage all your websites/clients with one Reseller Hosting package instead of going through the hassle of managing multiple shared Hosting packages. This also lowers your Web Hosting costs significantly.
Additionally, you can resell hosting as a value added feature to your existing business or as a separate entity.