Operating System Design/Security/Privileges
A great percentage of the operating systems that are used today have user account privileges. These privileges may be for security reasons, to make sure that someone does not leak internal company information out, or to protect the well-being of the computer from some people's incompetence with a computer, or to prevent grave mistake from crashing the system, or to stop system failure or hacking of a computer (internally or externally) through an account with restricted access to files.
For example, in Windows XP, there are two kinds of accounts: Administrator and Limited. When one has an Administrator account, they have virtually ultimate power, and can change system files or other users' files at will. On the other hand, a person with a Limited account can only change their own files and such. Multi-user Operating systems should incorporate user account privilege restriction in their design.