General OS security

It should be fairly obvious that security is important; you don't want someone to take control of a device and disrupt its normal functioning—imagine the havoc if someone could stop air traffic control systems or hospital equipment from functioning properly.

The importance of security to an individual machine depends on the context:

Part of securing a machine is identifying the level of risk. By classifying threats into categories, we can break down the issues and see which ones we need to concern ourselves with.