Google Assistant does a great many things and one of those is having the ability to read out your messages to you. For the longest time though it was only able to read messages that came in the form of a text from Hangouts or the actual Messages app. Now, though, it's just been recently updated to allow for reading messages from third-party apps making it more user-friendly to those that like to be able to use their message apps hands-free whenever possible.
In addition to reading your messages from the Messages app and Hangouts, Google Assistant now has the ability to read your messages from WhatsApp, Telegram, Slack, Discord and many other apps that you may use for social chat between friends, family, and coworkers.
This will only work for messages that are strictly text, so that's worth keeping in mind as you won't be able to have Google read off a message that is only an image or an audio clip of any kind. Google will tell you however that the message contains audio or an image so you'll know to go look at or listen to whatever it is.
Though it would be nice for everyone to have one single app for all of the chatting they do, this simply isn't the reality of things as many people use more than a few apps for all their communication. Google Assistant's new hands-free "read aloud" option for these other apps, if Google has privileges enabled to read notifications, you can ask Google to read any incoming messages and it'll read back the message as well as tell you who it's from.
What's more is that you can also reply back using Google Assistant if you want the entire conversation to be hands-free, though you will still have the option or Google read your messages and type a response if you wish.
If you have multiple messages coming in on a consistent basis and ask Google to read them all, you'll notice that those messages which pop up will have the icon for the app which they came from, so you have a visual way of discerning who is messaging you from where.
This update to Google Assistant seems to have started rolling out earlier this week and within the last few days, so it should be live for pretty much anyone that has a phone with Google Assistant on it, which is basically every Android pone out there these days. With that said it's also still possible that the feature may not be live for you yet.
The best way to check would be to have someone send you a message through a third-party app and see if Google is able to read it back to you. You'll know right away whether or not it's possible. Though in many cases it might be easier for some users to read the messages themselves, this will likely become very useful for those who have Google Assistant speakers connected to their accounts and placed throughout the home.