When do we use wedges and lines for VSEPR?
When we want to represent a three-dimensional geometry.
It is always difficult to represent a three dimensional object on two dimensional paper. A tetrahedron is a problem, so are trigonal bipyramids and octahedra. Now, when we represent a tetrahedron, say
Note that in order to visualize the geometry, do not embarrassed if you need a model. Such models are always permitted in a test, and would always be found on the desks of distinguished professors of organic and inorganic chemistry.