Replacing flashing can be costly and difficult, so many homeowners are reluctant to replace it when the roof is replaced.
When a roof is installed, a layer of tar is laid to adhere the shingles to the flashing. Over time, that tar hardens and crystallizes becoming incredibly difficult to remove. If shingles need to be replaced, that tar has to come off as well. It is virtually impossible to remove all of the tar without damaging flashing.