A. It is useful for diagnosing data dependent problems by filling the packet with the byte pattern following the "f" in the command line.
B. It is useful for flooding from slow character based terminals since it only displays a dot "." for each packet sent in fast mode.
C. It is useful for flooding from slow character based terminals since it only displays a dot "." for each packet sent and a backspace on returned in fast mode.
D. The command can only be run by root.