One of the nice things about sending traditional SMS messages is the option not to type them. Using the Google app (or an Android Wear device), you can just say "OK Google, send a text to Mom: Look Ma, no hands!" You can do the same through Hangouts and email. The feature is a life saver while driving, when messages would otherwise go unanswered.
Now you're able to use third-party messengers as well. Google has announced support for WhatsApp, Viber, WeChat, Telegram, and NextPlus. Just say "send a Viber message" or "send a Telegram message" in place of text or email.
Right now this functionality only works in English, but support for other languages is in the works.