Android O keeps multi-window apps open on the home screen when you back out of those applications. If you've got one window open playing a video on YouTube and then one window open having a conversation with a friend in your favorite chat app, with the Android Developer Preview installed you can back out to the home screen and the app windows will stay open on the home screen instead of minimizing to the recents button. That being said, the apps do still minimize to a smaller section of the screen, but they don't completely disappear this time.
In Android O the windows will actually minimize to a small portion of the display just under the status bar, so you'll still be aware of what's open anytime you glance at the window up at the top of the user interface. In addition to this minimization change, Google has even added in another visual option so users can choose how to display their open multi-window apps. While minimized you can pull down on the black bar just below the app window to open it up a little and see more details, but all without actually opening the app all the way and backing out of the home screen.
These are probably going to be pretty minor changes for the majority of users, but for those that use the multi-window feature quite often it's likely to be more than just a tiny benefit. Implementing this visual change to multi-window apps will easily be able to serve as a reminder for users to go back to the apps they had open instead of forgetting or just closing out of them because they ended up with other things to do. However useful this might be to some users though it's still important to remember that Google could make more changes to the way this new functionality works, or they could just remove it altogether if they don't feel like it's a good fit for the final version of the software. There are three more Developer Previews to be released before Google finally gets to a stable public release of Android O, so there's plenty of time for things to change.