There are two cases in which this body state would be desirable: the first being for epileptic patients to control their seizures, and the second, to induce
rapid weight loss. When the diet is altered so that very few
carbohydrates are taken in, the body must compensate for the energy deficit by burning
body fat for fuel as opposed to glycogen. This creates high amounts of
ketones in the blood when fat is converted to ketones, the compounds that fats are converted to so that they may be utilized as fuel. Many people experience ketosis for short periods on a regular basis, for example in the morning many people have exhausted their glycogen supplies overnight, inducing ketosis which is the culprit of many cases of the infamous "
morning breath."
This is useful for controlling epilepsy because when the body is in extended ketosis, the body is no longer using glycogen for fuel. The brain needs glycogen to run, it is one of the few organs that functions very poorly in a ketogenic state, all brain functions are reduced in this state. If ketosis is induced in an epilepsy patient, their brains will be less likely to react to a seizure trigger, thus making the disease easier to control.
For most of us, however, slowing of the brain functions is not a desired state, but ketosis can be very useful to someone trying to lose a lot of weight in a short period of time. Note the emphasis on short here, putting your body through extended periods of ketosis without being under medical supervision is a really good way to mess up your liver and kidneys. When the body is in continuous ketosis though, it is burning fat at an optimal efficiency. Also, appetite is reduced, which is responsible for that phenomenon that anyone who may have tried to fast for several days might have noticed, that after a while, you just aren't hungry anymore. Another benefit of ketosis for weight loss is that in this state, the body is less likely to turn to protein (read: muscle mass) for fuel, so the weight that you're losing will be body fat and not muscle.
Another thing that anyone attempting to lose weight rapidly should be aware of-- ketosis also causes an initial drop in water weight, which is not body fat and will be regained when normal eating is resumed. It's a lot easier to get dehydrated when in ketosis, so drinking lots of water is important. And beware of death breath: one of the compounds produced in this state is acetone, which gives nail polish remover its distinctive smell.