There are several advantages to the MUC protocol (Jabber is an IM protocol, MUC is the conference protocol):
1) MUC supports additional features, including file transfer and embedded presence notification (i.e. In Psi the online/away/idle/etc... status of each participant is listed on the righthand pane)
2) MUC can ride on HTTP, so it's easier to use when behind a firewall that blocks IRC.
3) The MUC conference can be "bridged" to the IRC, so users can participate in both while choosing which protocol they prefer.
4) Most clients will provide "popup" notification when activity occurs on a MUC, allowing the conferees to focus on other tasks when the conference is idle.
5) While GAIM connects well to IRC, not everyone wants to use the GAIM interface, this allows users more choices for how they want to connect.

Alan Dayley wrote:

As you may have noted from my previous emails, I am trying out the jabber 
conference.  So far I am not impressed nor disappointed.

Why is the jabber conference better than the IRC channel?

They seem to provide the same function and, running both channels at the same 
time in GAIM, I see no difference.  Both are just as easy to connect to (Once 
my confusion about registering jabber was over.).  Both provide a chat 
session.

There must be a compelling reason besides being "new" to use jabber over IRC 
but I don't see it at the moment.  Any thoughts?

Alan