It's to ensure that you, the user, know when the boot process has changed substantially, and that you have a simple way to get back into familiar territory.
If you want to get rid of Chrome OS and all its user protection measures entirely, that's possible with official and relatively standardized means. The open source firmware community provides documentation and tested firmware images for that (of course: no warranty), most prominently https://mrchromebox.tech/
ChromeOS tends to provide the best experience for anything that ChromeOS is able to do.
I want to be able to choose at bootup, and not have that configuration be "easily obliterated" as someone else described. The warning is fine. The way back is a bit too simple...
If you want to get rid of Chrome OS and all its user protection measures entirely, that's possible with official and relatively standardized means. The open source firmware community provides documentation and tested firmware images for that (of course: no warranty), most prominently https://mrchromebox.tech/