Why does the body need sugar?
Sugar can be converted into glucose, which then releases energy via respiration that the body needs for anything to move.
The most important sugar is glucose. It is made in plants through photosynthesis and stores a lot of energy. However, the energy is difficult to access, and glucose isn't very accurate or precise in terms of distributing it, so it is an inefficient energy source (though useful for storage).
In order to solve this problem the body undergoes a process known as respiration, where the energy from glucose is transferred through a series of complex biochemical reactions into ATP molecules, adenosine triphosphate.
ATP then can release smaller, more manageable amounts of energy quickly and easily to muscles, neurons, proteins, organelles and anything else that needs energy - which is everything.
We do not need sugars directly - you can survive without gargling syrup. The body can derive sugars from carbohydrates, like bread, pasta, and (most) cereals.