Defoko (the bot) does automated task such as posting the chat logg, show advertizement in threads, and repost stuff and uppdates happening in the utau wiki 2.0. In the beta stage of the forum she was also used to test the showcase.
As for why they used defoko as the name and character icon, it might be that she has become the mascot of UTAU, being the voice that it comes with. So it wouldn't be too unreasonable to use her name as a bot for a forum mainly dedicated for UTAU. This just just my guess though, for all I know they just wanted to call it something else than bot-chan.