Well, it's related to Graphics on SFML, but users can use it for several purposes. Shapes and vectors are math concepts independent from display concepts, it's all about geometry.
For SFML and probably most applications using it, it isn't needed outside graphics stuff. But I use vector2 and 3 for everything on my logics that needs vectors, and may use other shapes (polygons will be of great use) in case you make them independent from drawable.
I understand that it's not the main purpose of them, but since it's there, and since they don't rely on graphics, they can be used outside for the most varied purposes.