Question

why some object needed streamlined body

Answer 1

when cars and aeroplanes move at very high speeds, their motion is opposed by friction offered by the air molecules surrounding them . the friction of air produces what is called DRAG , which opposes vehicles. the same applies to ships and boats . to reduce drag automobiles, ships, and aeroplanes are given a special shape called streamlined shape 

Answer 2

Many natural organisms like fish and many things like cars and aeroplanes and ships are made of streamlined shape. Streamlining is generally done to reduce friction and resistence from air and water