In the absence of metabolic disease, the body breaks down fat to use the resulting energy. Think about it evolutionarily; "there's no food right now, time to jettison my energy stores"?
Ketosis is a pretty weird state brought on by lack of carbohydrates. The liver starts producing ketones, ketones rapidly breakdown fat cells to boost blood sugar.
It is mostly unrelated to diabetes as that relates to insulin response.
In the absence of metabolic disease, the body breaks down fat to use the resulting energy. Think about it evolutionarily; "there's no food right now, time to jettison my energy stores"?