I use the bip IRC proxy. There are many possible reasons why one might want to use an IRC proxy, but mine is productivity.
I need to be on IRC for work (I work for a distributed company), and I need to be accessible via IRC through most of the workday. However, being on IRC all the time is dangerous for my productivity. Other people can handle it better, but I have a really hard time resisting the lure of the IRC window, even if it is minimized.
However, it is not a problem for me to resist launching the IRC application. By using a proxy, I can easily close the IRC application when I need to concentrate, and then re-start it when I am done with the task. The proxy keeps me accessible to people via IRC: if anyone needs to talk to me, they can, and I will read their messages when I come back online.