I'll provide my point with a simple example. Think about your favorite PC RTS game (I think of titles such as Total Annihilation, StarCraft, WarCraft II, and Dark Reign). Imagine someone who is a master of the game, he's been playing his whole life. However, imagine that he's very old, and he dosn't move the mouse so well anymore. Will he win against some new whipper snapper who manipulates the interface incredibly fast? Probably not, and that's because these games are about the fast-action tactics, not about good strategy.
Of course, pure strategy games are all turn based, such as Chess, Go, even computer Strategy games such as Master of Orion. Turn based games can be slow and boring. Even if you love playing them, they don't get the adrenaline flowing like a good old game of TA. I argue that Herzog Zwei combined the best of both worlds by giving you a strategy game, combined with an action role. [For the record, the 3d games Battlezone, and Battlezone II did an EXCELLENT job of creating this type of game as well ]
The amazing thing that Herzog Zwei did (which Battlezone I/II replicated), was to give you your own command unit that you had to use to interface with the game world. Instead of being the omnipotent mouse-toting god, you are piloting a unit around the world. Your unit moves at a given rate just like all other units. If you want a unit to do something, you need to get involved in commanding it, show it where to go, that sort of thing. The result is that the rate at which you can control the other units is slowed down, turning it from a tactics game into a strategy game. Meanwhile, you take a much more active role with your unit, giving you back the fast action which makes real-time games fun.
Now that you know why Herzog Zwei was neat, read the full game description.