Today, with no formal vendor interface, a lot of code across Android needs to be updated when a device moves to a newer version of Android. And with the vendor not updating, there is no, or a very delayed, Android update.
With a stable vendor interface providing access to the hardware-specific parts of Android, device makers can choose to deliver a new Android release to consumers by just updating the Android OS framework without any additional work required from the silicon manufacturers. Project Treble will be coming to all new devices launched with Android O and beyond. In fact, the new Project Treble architecture is already running on the Developer Preview of O for Pixel phones.