Some object needed streamlined body. Many animals like birds, dolphins and many machines like airplanes and submarines have steamlined bodies. This is to reduce fiction drag as they move through either air or water. To explain you better let me give you an example of birds which have boat shaped (steamlined) body which helps them to float in the air easily, it also make their body lighter to move on air and even water. It also occurs when the liquid closest to the object sticks to its surface, exerting a force that is opposite to the motion of the object.