For example, in space three-dimensional affine space generated by two skew lines is all the space three-dimensional, since they are not coplanar. For this reason it is not worth the Grassmann formula, which in this case would say that the space generated by the two straight lines has dimension 1 +1-0.
The affine geometry is intermediate between the geometry of vector spaces and the projective subspaces in a vector space are forced to pass through the origin. The affine space is then built in order to remedy this lack unnatural, but in doing so you lose the Grassmann formula, and many problems will lengthen the list of cases to consider: two straight lines can be accidents, coplanar, skew ... The projective space eliminates re-adding phenomena of parallelism of "new points at infinity", without restoring a "vantage point", and so here is Grassmann.
Could you help me understand this concept? In a strict and intuitive way.