Why is there more sun exposure on south and west sides of roof?

[Henry Holt]

I am reading J Scott's book on House flipping, and one of the comments in the book when he discusses examining the roof is as follows:

"South and West faces of roofs are generally most prone to wearing out, so these should be the parts of the roof that are examined most closely."

This question stems purely from being curious, by why do the South and West faces receive more sun exposure?

I figured it would be that the East and West sides of the house would receive the most direct sunlight, because of the sun rising and setting in those directions...  What am I missing?