Moss can and will hold and retain water, there for causing extra weight and degradation to the shingles. Some other reasons. Your home owners requires it, you want to be good on maintenance, piece of mind, aesthetics, etc. We have taken over 800 pounds of debris off of single resident houses. When that stuff gets wet, it weighs a ton. Just look at some of our photos. 800 pounds of wet slop just sitting up there cannot be good, in fact I have seen it root itself all the way through the shingles and into the wood. At that point, the roof just breaks apart like dust, like something you would see in a mummy movie.