Roofs get more than just rain water on them. Depending where you live, you may find moss and lichen growing on your shingles.
Roofs with moss and lichen growth need to be cleaned. Neglecting to remove the growth will only damage your roof further.
However, there are some risks to consider. You need to be careful to only step in dry areas of your roof, as wet algae is very slippery. You don't want to fall from your roof.