Main advantages are:
1. Accessible to those who don't have expertise or interest in setting up their own IRC bouncer
2. Usable from any web browser
3. More friendly UI
4. Usable by sites that want a chat room for their users (look at twit.tv -- embedded IRC client front and center).
There is no proprietary backend; the whole client, and the library powering it (node-irc), are open source.