They are specifically relevant to trying to enter a fasted state.

The presence or absence of ketones in your blood can tell you whether or not (say) that coffee you just drank took you out of the fasted state (that you were in for not eating overnight).

Whether or not you are in a fasted state matters not just for weight loss, but also for things like longevity (when you are in a fasted state your body cleans up dead cells that contribute to things like cancer, whereas in a non-fasted state it leaves them lying around).