Riding

People say that instead of going up and down, basically grind on him, and go back and forth. How does that help anything? I get the clit stimulation part but he isn't going anywhere. I haven't done this position yet and it kinda confuses me and makes me nervous. If you grind he doesn't go anywhere, so how does it help him get off and you for that matter if you're just sitting on him? Someone please help me get an explanation.