Android OS updates can take ages. The last time I updated my (admittedly now vintage) Motorola Droid Turbo, the phone was unusable for nearly 45 minutes while it slowly shuffled around system files. For that and other reasons, Google created a feature in Android 7.0 called Seamless Updates. Soon, with Android 8.0, Google will be updating that feature to "Streaming Updates". Not only will it eliminate update downtime (as Seamless Updates can already do), it will also discard the user-area free space requirement.
Seamless Updates split the on-disk system partition into two separate spaces: one "online" space for the currently-running system, and one "offline" space for pending updates. The idea is that the phone could update critical system files while the machine is running without having to unmount them, allowing the user to continue working (or more likely, playing.) Once the update is done, the device can reboot, swapping the "online" and "offline" partitions and booting from the formerly-"offline" space. If there's a problem booting on the updated system, the device can simply drop back to the pre-update system and try again.
The Seamless Updates feature is pretty nice, but it doesn't tackle what is for some users the biggest hurdle to OS updates: free disk space. The update data still has to be downloaded to the user storage area, which means that the device will likely need 1GB or more of free space. That's not that big a deal for folks like you or I with devices packing 64GB of memory, but for cheaper devices with only 16, 8, or even 4GB of flash on-board, it can get irritating.
With Streaming Updates, Android 8 devices that make use of the feature will download updates directly to the "offline" system partition. Not only does this make updates faster—no more time spent shuffling system files—it also eliminates the free disk space requirement. Google says users will need around 100 KB of free space for metadata, and that's it. The company implies that removing this limitation will lead to more users performing updates, and thus less vulnerable devices hanging around.
Streaming Updates isn't a new idea. Google already uses the system in its ChromeOS. However, the only Android devices to have shipped so far with the Seamless Updates system are the Pixel and Pixel XL phones. Unfortunately, that means that as of right now, these are also the only Android devices which can support the new Streaming Updates. Hopefully when Android 8.0 devices hit the market they've taken advantage of this massive improvement in the Android update mechanism.