1. To more easily imagine that you are Captain Haddock in The adventures of Tintin.
2. To have something to stroke sagely when the hair is gone from on top.
3. To catch dribbles of ale and other liquids.  The benefits of this increase with age.
4. To advertise your affection for small mammals.
5. To become more august and presidential, at least in a nineteenth century sense.
6. As a musician, to more convincingly pull off your move towards Americana.
7. To blend and knit together facial hair and woolly hat into unbeatable protection against winter.
8. Obviously, to hide a weak chin.
9. Conversely, to hide a chin the size of Jimmy Hill’s.
10. For the ultimate pleasure of shaving the bloody thing off.