You can get nasty problems if the edge felt near the guttering is exposed to the weather. The felt rots away leaving holes where birds and water can get into the roof space. In this case there were birds nests in the loft and dead birds which caused maggots to appear in the loft.
This was repaired using a new plastic under felt system. It also comes with a bird comb attachment, which stops birds getting into loft and causing damage. It will also stop bees or wasps getting in and nesting.