The 3rd party sells their service, which is web-based chat application, to any company. Think of it as a company that offers hosted forums (SaaS model), but chats instead of forums. It allowed Youporn to offer a chat service to users, without us having to develop and maintain it. Again this decision was done before we took over.
Using chat.youporn.com is not that different than using ypchat.com, personally I would have preferred the latter for obvious reasons.