Using a hot tub cover is important, whether you have an indoor or outdoor hot tub. The two basic reasons are to keep dirt and debris out of the water and to trap the heat, saving energy and reducing the cost of using the tub. The kind of cover you use depends on where your hot tub is located, the weather conditions (if it's an outdoor spa), and how much you have to spend.
Soft covers are the most affordable and popular style. If you have an indoor hot tub, a quality soft cover is probably all you need. The exception would be if the spa is in tight confines and you need to be able to walk on the cover. A hard cover is the correct choice then, especially if the only way to get from one end of the room to the other is to go over the tub. Just be sure to get one that is rated to handle the weight of the heaviest person who'll be walking on it.
A soft cover can be used with outdoor hot tubs, too, but some care should be taken to get one made from a rugged, tear-proof material. Thick, marine-grade vinyl is the most common material for these types of covers. Be sure to check your tub often when it's covered in order to remove debris that can weigh it down, wear it out faster, and reduce the heat-trapping efficiency.
A hard hot tub cover is generally more expensive, but often provides superior energy savings and long-term efficiency. It's also more convenient because many of them can be walked on.
Depending on the size of your hot tub, hard covers will often be very heavy and will require a mechanism to place and remove it. Soft covers are more convenient in that they are lighter, but they can still be a bit heavy. If you choose a soft tub cover, consider investing in one of the many cover removal aids on the market. They are very handy and might even save you from a hernia or pulled back muscle.