It has a very useful purpose in scripting: in order to discard unwanted (diagnostic or otherwise) output from a command in a shell script, one simply redirects this output to /dev/null.
One example is using grep to see whether a file contains a regex; grep will print the line(s) containing this regex, which may not be needed in the script--one is interested only in yes/no
if grep REGEX file >/dev/null 2>&1; then ...
will do nicely.
/dev/null is also an empty file, and can be used to truncate existing files by copying it on top of them
cp /dev/null file-to-truncate
will in effect make file-to-truncate to be of size zero (even if it previously existed), because it copied an empty file (there are other ways of doing that)