Question

why does aeroplane have streamlined structure

Answer 1

anything that moves through a fluid, be it a plane, a swimmer, a bird or a submarine, can do so but at a cost. it takes energy.

streamlining reduces the amount of energy required. the enemy in this is "drag".

to take an example, if a jet plane had big square leading edge, it would effectively be a sail in reverse, with much larger drag than necessary.