Any thing or part 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 significantly optimized by finding their optimal-performing 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 make it even more difficult, 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.