During the "macro" portion of this inspection (looking at the home as a whole) I initially thought that the roof on this home had simply been neglected, hence the egregious amount of moss and lichen that was growing on it. Upon closer inspection it became apparent that the shingle roof had been installed over the top of older asphalt shingles.
When properly installed, wood shingle roofing is applied over skip-sheathing, or a modern equivalent. This allows the wood to "breathe", reducing the amount of time that moisture is in contact with the wood. Without the skip-sheathing the moisture remains in the wood creating a perfect environment for rot.
In the second picture you can see the asphalt shingles directly below the wood shingles. Why this was done this way is simple; it cost less, was easier and took less time. Whether this was a DIY job or the shoddy work of a lazy contractor I will never know.