SQL Always On COPY ONLY backups - what's the point if I cant restore the AG from these backups?
Copy only backups are generally used for situations where you need to restore the data for an unusual reason without disrupting the current backup chain (assuming there is one). Typical reasons are restores to dev boxes to test code or look at an irregularity in the data. You get the idea.

As a side note, taking backups from AG secondaries is kind of a donkey. If the AG is behind at all, your backups are behind. If you're not already using the secondary as a readable replica, they make it an expensive behind-backup device since now you have to fully license it (assuming you have software assurance, and all that).