The so called Net representation for a Tetrahedron is depicted in the following image ( link to wolfram ) :
What is this for ? How to reason about this and how to generate this very same representation for any polyhedron ? There is an algorithm or a formula for that ?
There is a good article about the nets of polyhedra and about the problems of generating it: http://www.ams.org/samplings/feature-column/fcarc-nets In short: The main problems are that they are not unique and that there are cases where you can get overlaps.
On why using nets: It is a convenient 2D-representation (but not unique) of 3D-Objects for certain purposes, and it is just one of many representations. This one is especially convenient for working with the angles of the faces which you cannot get so easily out of a (3D to 2D) projection.