> Isn't that the whole point of fat storage, to provide caloric fuel when you don't have access to it through food?

The whole point is to use it as a last resort to not die. Not to just use it up when you could eat normally. And there are a lot of reasons why this is the case.

First, it's what the body and your brain wants, and they make you feel like absolute shit if you don't eat. What you weigh doesn't matter, starving yourself is really really hard, because every cell in your body is screaming at you to not do it.

Secondly, it's unhealthy. Part of that can be alleviated with supplements as you say, but the other part is that your body goes into survival mode when living off your fat reserves. Your body weakens, you health worsens, your immune system goes downhill, you have less energy, and some of these effects persist even after you start eating normally again.

And then there's the problem that such a crash diet, even if you go through with it, may make you lose weight, but it won't keep the weight off. Heck, you just taught your body that "sometimes there will be no food for 6+ months", and to that, the body responds "well, then I'm going to eat literally everything I can get hold of while we do have access to food". The body tries to repair the damage and recover the lost fat reserves. So you end up regaining the weight you lost, but with some additional permanent damage to your health and to your metabolism.

All because you had this idea in your head that literally anything, including month-long starvation, was better than being fat.