Well, it's controversial in my opinion, since we're already funding the game by investing reds into whatsoever the resource boosted, I think it shall be removed, but I ignore the reason why it's kept anyway. My best guess would just be a way for the kind supporters not really playing an era pressing on that boost option and donate the fixed amount of reds to BattleDawn. You shall PM a staff member and you will get the best answer from them. I can't say much about it.
Cheers!