You've done it a million times: You're using one app, like Twitter, when you want to pop over to another (your texting app, or Spotify maybe) to do something real quick. Then it's back to Twitter.
Hopping to another app and back again is so common, Google made it easier in Nougat. If you double-tap the recents button, it will skip showing you the recent apps and just instantly pop you over to the last app you had open. Double-tap it again, and you pop back. Once you get used to it, it's a real time saver.