The angle of the wood on the outside rows makes the pile more stable. It is also possible to make the pile wider as you go up.
By angling the outside rows as shown in this picture, you make the pile lean in on itself. This is a very stable configuration. As the pile gets taller, it also gets wider. This helps the lower logs stay dry in the rain. Finally, the middle row is taller than the outsides. This helps the tarp make a sloped roof over the pile to move rain water off.