This is about people accidentally pressing keys on the wrong side of the keyboard. It makes sure that those people are not interrupted in their writing, while still presenting a clear image to the user that is not confusing. It’s a line of defense, like the buttons that are actually a bit larger than they are displayed – just in order to make sure that users missing those buttons is less likely.
It’s error correction. The feature is not that there are repeated buttons on the keyboard and users can pick which they want to use. Users are not supposed to use those. They are not supposed to know (or have to know) about them.