what's the advantage of fish having a streamlined body ?

The meaning of streamlined here, is that the the fish's body is narrow in the front and the back, but broader in the middle. This helps the fish the cut through the water as the fluid friction is opposed by this. 

 A fish with a streamline body could cut water easily and move faster. As, you may know that streamline body reduces drag friction of fluid.

Friction produced by fluids are known as drag.Water produces drag.As fish is an aquatic animal,it should overcome drag.Streamlining is one ofd the better methods that reduces friction.A  streamlined body is sharp at the front and rounded at the back.Thus the streamlined body of a fish helps the fish to reduce the drag and to move easily and faster through water.

due to this steamlined body the fishes can move easily in water.this help to reduce drag...

fishes have a streamlined shape to lose less energy in overcoming friction.