Suspension seat posts are great for taking the ouch out of rides on dirt roads and rough pavement. They do this by providing about an inch (depending on the model) of sprung travel so you literally float over impacts that otherwise beat up your body. That's why most riders who use shock seat posts swear by them and praise how they eliminate most if not all, seat discomfort, lower-back pain and fatigue associated with demanding rides. Using one, you should finish rides feeling a lot better and you should be able to pedal longer with the same effort.
These posts are also an inexpensive way to add rear suspension to a bike that's already got front shocks (far cheaper than buying a new dual-suspension bicycle, too). It's the perfect use of a suspension seat post because you've probably grown used to having some impact protection up front and also become tired of not having any in back. You'll appreciate having some cushioning. And, though shock posts don't provide the amount of travel and control a true rear-suspension system does, a post is a simple comfort device that's unlikely to break down and that won't add much weight to your bike.
The best way to see what these posts offer is to try one. Because there are different seat post models and because each bicycle requires a specific diameter, you need to let us know what bike you're upgrading. We can then show you a compatible post and demonstrate how it works.