I love ground work. The horse can learn new things without worrying about the rider on its back.. sort of how lunging with us on is great for our riding since we don't have to focus on the horse, just our bodies.
For me, groundwork is working in a halter and leadrope on the basics of leading, learning to disengage and yield certain parts of the horse, helping the horse overcome a fear or "mental-obstacle" via desensitizing and sacking out.
Lunging can be part of ground work but I consider that separate. And that's mainly a substitute for riding. Riding the horse has the rider to worry about, lunging the horse can work on its own gaits and tempo and get exercise. I like to keep those sessions short and sweet.
"Strength is the ability to use a muscle without tension"