A billing system provides an easy way to manage how you bill your clients. Creating invoices or repeat billing clients each month can be entirely automated with this software. It can also automate setting up accounts under your reseller, as well as automate the process of sending welcome emails and so on to your clients. The industry leader in this space is WHMCS ("WHM Complete Solution") and this product is owned by cPanel. It's paid software so requires a license, and monthly fee is dependent on the number of active clients in your install. You can find out more about WHMCS here https://www.whmcs.com/