The black streaks you may see on your roof is algae. It is eating the limestone in your roof shingles. Once the algae has become visible, the stains and damage will only continue to get worse. If left untreated, the growth will hold moisture within the shingles causing premature aging, rotting, and granule loss.
A dirty roof can lead to the premature failure of your roof. Maintaining your roof will extend its life, saving you the replacement cost.