Anything, or part of anything that is exposed to a moving gas or liquid can benefit from a streamlined shape.
In particular, wings, fins, ducts, propellers and turbines can all be vastly optimized by using the optimal correct shape.
"Optimal" is complicated; it depends on size, speed, turbulence, altitude (or depth), the properties of your medium (temperature and density) and the surface finish you use (rough or smooth).
To further complicated things, many of those things are a range (e.g. speed, temperature, pressure, etc) and when they are, there's often one point in that range where you need to concentrate on optimizing (e.g. cruising speed at cruising altitude).
Airfoil Tools does this optimization for you.