Good fences make good neighbors. When trying to secure a home or computer, separating the areas where resources are and people work prevents accidents and loss of data or private information. We are preventing the information worlds from colliding.
Newbie: Think of domain separation like not wanting to share a room with your siblings. You don’t really have a desire to share your business with them, and they probably don’t want you knowing theirs. To sum up this domain, our private information should not be used in an environment that is shared with others.