Let me give you an example:
Your driveway has got moss all over it and is old and has a few cracks from age over time. With the moss/mold/lichens and whatever other goodie may be growing on a driveway. They will hold moisture on the surface longer. If you're unlucky you can then get stuff starting the grow in the cracks due to the increased moisture being held on the surface longer. Once this happens, the cracks can potentially get bigger over time due to plantlife etc trying to grow in them. If the cracks go all the way through then you can even get tree roots coming up from underneath if they have a regular supply of water coming down through the cracks.
If you have a steep driveway with sepage going across it everytime it rains, it can turn it into a death trap with how slipery it could get as well.
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