Updates to the Chrome browser build for ChromeOS came out later than the standard Linux/Windows/Mac Chrome builds because the special chromeos build needed time to be integrated/tested with the rest of ChromeOS.
With LaCros, the browser can be updated independently from the rest of the OS.
I remember when - and miss - the OS being "a single Chrome window, that's the shell, that's it". It was simple. Unbloated. Now it's an absolute mess that Google doesn't know what to do with - treat it like an Android device, or treat it like a computer.
this presumably allows them to ease that a bit and apply some updates with the system still running, with maybe a quicker browser-only restart
This has been the opposite of my experience on the stable channel. Perhaps you're on one of the more volatile ChromeOS update channels? Or you use your device infrequently and put it to sleep when not in use, resulting in a similar UX issue that plagues game consoles: the appearance of always having updates when used.