asterisk users - Apr 2004 - calling in for Voicemailmain - what does "o" do?

The wiki pages say: Users typically want to be able to call their own
telephone number to pick up voice mail. To enable them to do this, include
the following entry in extensions.conf. 

exten => o,1,voicemain
Note that this extension starts with a lower case letter o. 

But what does this really do? If I've set up the dialplan to go to voicemail
if the channel is busy, I go to voicemail anyway. 

Paul Mahler