/dev/null - Use to send unwanted output of program

Example:
$ ls > /dev/null
Output of above command is not shown on screen its send to this special file. The /dev directory contains other device files. The files in this directory mostly represent peripheral devices such disks like floppy disk, sound card, line printers etc. See the file system tutorial for more information on Linux disk, partition and file system.

Future Point:
Run the following two commands

$ ls > /dev/null

$ rm > /dev/null

1) Why the output of last command is not redirected to /dev/null device?