And to end any possible doubts about the results of MusicBee's implementation of RG128:
I ran several sorts of audio (classical, rock, spoken word) through MB's volume normalisation with setting the slider to '0dB', and these are some representative results. (perfect -18LUFS)
Yet I am afraid all above technicalities might also confuse many new users, so it might be sensible to summarize a little bit:
Setting the slider in MusicBee to '0' will indeed roughly match the old ReplayGain1 default value.
So the answer to your question in my opinion would be:
The 0dB position mainly represents and suggests some compatibility with an old standard.
The currently implemented R128 algorythm is yet way more sophisticated then the old RG1, giving far better results for all sorts of music, audio books, movies/commercials, etc. etc.
In broadcasting nowadays the advise is usually to aim for the region of -23LUFS.
So be aware that the '0' position in MusicBee is (in my opinion) not the advised setting if the rest of your audio chain and media are up to it and are set up well.
Therefore I personally would advise a setting of somewhere in the region of -5dB.
(apologies for repeating myself:
http://getmusicbee.com/forum/index.php?topic=10394.msg73947#msg73947)