Ketones are used to fuel the processes that keep cells alive and allow them to perform their function.
Ketones get into cells through a special transporter protein (monocarboxylate transporter) that is widely expressed. Ketones can cross the blood brain barrier, and so are used as a fuel in all tissues apart from the liver.
The breakdown of ketones to release energy follows the same metabolic pathway throughout the body. BHB is converted into Acetoacetate, which is processed to form a key metabolic intermediate called Acetyl CoA. Acetyl-CoA is a convergence point for the breakdown products of carbohydrate and fat, as well as ketones. Acetyl-CoA is used in a process called the Krebs Cycle (or Citric Acid Cycle), that happens inside of the mitochondria. The Krebs Cycle is a key set of reactions that converts the chemical energy in food into a form that our cells can use to do work.
As well as being a fuel, ketones can have helpful signaling effects.
Learn more here: https://hvmn.com/blog/ketones/how-are-ketones-used-in-the-body