Ketosis occurs when the body has not enough glucose to function (due to the complete removal of carbohydrates from your diet). When the body has no glucose it instead metabolises fat to provide energy and releases ketone bodies into the blood stream.
This places you in ketosis and it usually takes three days following a ketogenic diet to reach it (though you can speed things up using exogenous ketones).
The first people that benefit from ketogenic diets (and being in ketosis) are those who neurological disorders (Parkinson’s, Alzheimer’s) and people who have suffered head injuries/multiple concussions (sports people, accident survivors). Ketogenic diets are also incredibly effective for people with epilepsy.
Ketosis can benefit people with metabolic diseases, particularly Type II Diabetes. Studies have shown that it is so effective that many sufferers have been able to stop taking their medication because they no longer needed it (obviously that doesn’t mean you should do so without getting approval from a doctor).
Ketosis may also help you to live longer, this is because caloric restriction has been associated with increased longevity. Last but not least, ketogenic diets and ketosis can absolutely help you with weight loss and weight maintenance. This isn’t necessarily because the ketogenic diet is better than any other diet, but because you are 1) being careful about what you eat, and 2) staying in a calorie deficit which will lead to weight loss.