When roofing materials are designed they are tested against normal elements. These are wind, rain, UV Rays and acid rain. They do not however test for moss and debris collecting on them. In fact most roofing manufacturers put exclusions in their warrantees if the roof is not maintained. This is because of the prior issue of water diversion. But also the erosion that moss and debris can cause. Especially for asphalt roofing shingles.
When moss grows on the roof it’s tiny tentacles can grab and dislodge the granules on top of the roof that reflect the UV rays and prolong the life of the roof.
When debris is left on the roof it can enhance the growth of moss. It also traps moisture which causes loss of granules as well. For wood shakes it promotes decay of the wood.