colon command

I knew colon command from a bash script which one of my colleagues wrote. He uses it because of usage as here document like

:<<doc
usage: foobar <arg1> [arg2]
Change arg1 as args, if arg2 is empty...
option:
...
doc

The colon is a command to do nothing and return exit code zero. That's OK. I understand.

I don't know why it is used here. I'll ask him today.