Deserts are generally useless when unimproved, but can be beneficial if strategically improved.
Deserts produce 1 resource. Irrigation adds 1 food, mines add another shield and road adds 1 trade arrow.
Oases can be found in deserts, generating an extra 3 food.
It costs 1 Movement Point to travel over a Desert region, and you get no defensive bonus.
Excessive pollution in the world can cause plains and grasslands to become desert.