Many of our kayaks (Tercels and Hobbys) have standard seats in which the seat height is fixed (unless you put a pad on it), however the Viper 51s and Viper 55s have platform seats that can be raised and lowered as well as being moved forward and back.
Most of us don’t consider changing the height of the seat but raising the seat can have a number of advantages: If you sit higher in the kayak, you can deliver more power into the water and paddle more efficiently. Having your pelvis a fair bit higher than your feet can also alleviate back pain and allow you to adopt a better, more upright posture. If you paddle a Viper 51 or 55, you could also consider pitching the seat forward a bit so that the rear of the seat is higher than the front. This again helps you to tilt forward on your sit bones and adopt a more positive, powerful paddling posture. Mark Giddings (Mark Gees), the well known maker of exquisite wooden kayak seats, has told me that all his seats are designed with a forward pitch for this reason.
The downside is that raising your seat will reduce stability as you raise your centre of gravity, so it can take a bit of getting used to. However, if you’re looking to get used to paddling a faster (less stable) boat, then raising the seat is a good, incremental, way of adjusting to a less stable platform.