If you tap on the Messages app icon, you will probably see your most recent text message conversation. But if you 3D Touch on the Messages app, you will see a list of names of folks who have recently had text message conversations with you. Assuming that you wanted to send a message to, or review a recent message from, one of those three people, this is a faster way to jump directly to the text message conversation with that person.
If you’re like me, 3D Touch is a feature you intend to use more but never do. Maybe this article by Jeff Richardson is the impetus you need. Jeff describes several handy ways to use 3D Touch. If nothing else, make sure you heed the tip for using 3D Touch to move your cursor when typing.