how to use an spline command?
Sometimes the required geometry is not properly constructed with lines and arcs. We use splines through x,y points driven by a spreadsheet to create blower housing scrolls.
Splines can also be used to replace several entities with a single spline entity such as with the "fit spline" command. I've used this to reduce the number of surfaces in a model in order to get surfaces to blend together better, sometimes more "patches" in a surface can be problematic for modeling.
In the picture, both housings are lofted sections driven by a spline to achieve the scroll shape, and the housing on the right has been "simplified" from a surface count perspective by using "fit spline" for the loft section profiles.
These are just two uses, we also use them for airfoil shapes, for making smooth, tangent blends.
Splines are for when you want real smooth curvy stuff!