This way, new users who get Ubuntu, and open Pidgin, see #ubuntu, and can click on it, to chat about Ubuntu and get help if they wonder something.
This could also make it easier to get people involved in the community.
To make it easy for them to get help and to help others.
To make them feel that they are not only using Ubuntu, but that they are actually a part of it.
And hopefully also get some contributors. :)
So if they get involved in the community, and feel they're part of it, they will be like "Wow, I'm in this cool Ubuntu club!" and be proud and go brag about it. :D