Just because you own a dressage horse, doesn’t mean, the only thing it can do is dressage.
Expanding the horse’s capacity is just as important as improving a certain disciplin. Besides, their’s a lot of great benefits of for example going on a trailside, doing polework, agility, round-up and so on. Horses doing more than just one specific thing, are actually better and faster learners. 

I took Demi out for an agility lesson this Saturday and he absolutely loved it! I could really feel how he enjoyed doing something else, than just going for a ride. Besides this might help me, getting a more relaxed horse when handling him from the ground, which is really needed. This is couldn’t obtain, if I just rode him every time I was around him.
Groundwork is extremely important and necessary, when handling horse, especially with a nervous and curious horse like Demi.