Off-road cover is an optional feature you can add on to your insurance policy. This feature is primarily for anyone prone to off-road driving. Off-road insurance usually covers any damage that has happened the vehicle while being driven off-road. This cover is more common in 4x4 jeeps that promote dune bashing for example.
No, you do not need off road cover. However, if you regularly drive off-road then it is highly recommended to take out this feature. Your insurance provider may only provide this cover if they deem your vehicle as capable enough to withstand off-road driving.