The dropper post allows you to do two things at the right seat height easily, rather than stopping and having to loosen and drop your post manually.
When climbing, or riding flat, you want your seat higher so you get good leg extension and balance.
When you go downhill, especially on technical terrain, you want to drop your seat (and center of balance) so you don't go over the bars.
That's the basic concept.
Set your dropper so its at the best height for riding when extended (high). Then you get whatever drop it allows.
Once you start riding mixed terrain with one you will like it. A good example I did last year was Slickrock Trail in Moab. Used that dropper a lot as the route was like a roller coaster of ups and downs!