Shingle construction starts with a fiberglass intermat. During production the fiberglass intermat is saturated with asphalt and dried repeatedly until the desired thickness is achieved. This intermat is not a waterproofing layer but is necessary to keep the shingle from cracking and separating. Over time the shingles surface wears down. Eventually the intermat will become exposed. It is generally accepted that when this occurs it is time to replace the roof. Without the fiberglass intermat holding the shingle construction together the shingle cannot longer withstand thermal shock (expansion and contraction) and will crack, possibly allowing seepage. You may want to consider replacing the roof at this time.