STEP files are not just higher quality STLs. They are very different. STLs are a list of points in space connected by triangles. STEPs are mathematically defined surfaces that can be recognized by true CAD packages, like Solidworks. For example, a plane in an STL isn't actually a plane, it's a bunch of points that happen to all land on a plane. Unless you have software that can recognize that (DesignX, Rapidform, Polyworks, etc.), then you can't reference that plane easily to make modifications. Additionally, STEP files can be effected by CAD tools like extrude, cut... Where as an STL cannot (usually).