Pointers-to-pointers are not special in this regard. The answer for them is the same as for any other pointers:
It tells the compiler how to interpret the pointed value, when you dereference the pointer.
(When you do
**p, how many bytes should we read/write from the address stored at*p? And how should those bytes be interpreted?)It adds some type safety - you can't convert to and from a pointer to a different type without a cast.