On top of what others have said, one case where I've seen value in them being separate is to workaround problems in long gaps between deployments. A bug might

On top of what others have said, one case where I've seen value in them being separate is to workaround problems in long gaps between deployments. A bug might be Major or Minor but there is a high risk to fixing it. You might want to do it early in the development process, ahead of higher severity issues, so you get the most runtime on the fix before the next deployment.