a reseller plan gives you the opportunity to start your own hosting company. It also gives you a control panel called WHM. WHM stands for Web host manager. In this control panel, you can add clients, remove clients, add e-mail accounts, limit client disk space usage, limit client bandwidth usage, etc.
You really don't need to start your own hosting company to have a reseller plan. Because if you look at it a reseller plan sometimes offer a lot more than a shared hosting plan does. A major difference between the two is that with the reseller plan you get the WHM control panel and cpanel control panel with a shared hosting plan you only get cpanel.