In other words, it's to stop the developer of fakegoogle.com using your web browser to access google.com as if they are you.
Therefore, if you've written an app which will only ever contain cookies and authentications you've permitted, and only ever access servers that you've specified, then yeah, sure, CORB is irrelevant and you can safely ignore them. You and the malicious coder can both write code that runs on Android or iOS safe in the knowledge that there's an absolute sandbox between them - nothing the malicious coder ever does will ever leak your user's secrets to your server.
Likewise, if you're a malicious coder and you convince the user to give you their legit google.com secrets, you can safely send them wherever you want.
If you're writing a web browser equivalent app, that will run arbitrary code from untrusted third parties and store and release private or secret information, CORB makes sense and you should pay attention to the CORS headers.
CORB is solving a problem inherent in web browsers - they run untrusted code and code can cause your secrets to go to your server. It is annoying to you as a web app writer in the same way that locked doors are annoying to plumbers. It would be so much easier for the plumber if they could just come around to my house whenever it's convenient. And it doesn't solve any problem the plumber has (if some random steals all your gold, how is that the plumbers' problem?). Inside your house you don't lock every door. But you do lock the doors that separate trusted and untrusted people, and no amount of difficulty to plumbers will change that.