what is the importance of having sugar in your body? like, if your blood sugar is low but you don't feel hungry, what is the harm?

Sugar is the "fuel" that your body uses to function.  If you have no sugar - all body functions stop.  The brain, heart, and lungs can not function without sugar to fuel them.

If sugar levels go low enough, a coma happens.  If levels continue to drop, death can happen because there is nothing to fuel the body.

For someone who is NOT diabetic, this will rarely happen unless they have some other very strange or serious medical issue.  If an otherwise healthy person stops eating or skips a few meals, they might get "low" blood sugar, but the body will NOT allow it to go dangerously low.  The liver can release glucose reserves and the body can start burning fat and muscle to make more glucose.  (a person with severe anorexia might experience a dangerous low blood sugar issue.)

BUT - for someone who IS diabetic, their medications would be the most likely reason they are going too low.  For a diabetic, their body might not be able to release glucose fast enough to keep up with what the medication is doing to them.   A diabetic that has taken a little too much insulin would be in danger of having a problem from low blood sugar.

You can slip into a coma and die. 
That is why most insulin dependant diabetics carry a quick sugar source in case the misjudge their insulin needs.

Add: it is important to seperate blood sugar from dietary sugar intake.
Your body naturally converts various healthy foods to blood sugar.

The problem is that the body isn’t super efficient at burning nothing but fat as fuel, which leaves many of us feeling sluggish and performing worse in the gym.

It’s all down to personal preference when it comes to low vs high-carb, so I will instead focus on the other three macronutrients.

Protein, fiber and water.

Both protein and fiber are very satiating per calorie, and takes a long time to digest, meaning that you won’t be hungry an hour after eating, like you might be if you eat a bowl of cereal.

Protein will also lose about a quarter of its calories due to the thermic effect of food, which means that about 25 procent of its calories, give or take, to break it down, meaning that you get to eat even more.

Fiber, too, is very satiating, and only has 2 calories per gram, which is half of what carbs and protein contains, and less than a quarter of what fat contains (a dense 9 calories per gram).

Most vegetables are pretty high in fiber, and so are lentils and legumes. Grains like oats have a fair bit too, but usually less than the former food groups per calorie.

Talk with your doctor about this if you're concerned.

If you're not a diabetic, you'll burn reserves in your liver or start burning fat. That can go on for some time. But if you're diabetic, low blood sugar can make you crash and fall into a diabetic coma.

Sugars are ALL toxic to the liver EXCEPT lactose and galactose.   There is NO NEED for any sugar in the human diet. 

What glucose the body needs is made by the liver out of other things eaten. 

I don't expect you to believe me because you are addicted to fructose, the WORST sugar that actually kills. 

Sweet dreams.