1. Principles

Even on a standalone (not connected to a network) single user Linux workstation, different things happen in the background. Just turning the computer off may interrupt important operations and leave the computer in a problematic state (or at least some subsystems). Turning the computer off may cause problems, but in most cases it won't. It really depends on what is going on at the precise moment the power is removed. Here is a small list:

• Some editors or office applications are running in different windows or virtual consoles. Unless you tell them, they won't save the current document.
• You have just saved a large document. Linux has not yet written the document to disk even if the editor was told that the document was saved. This write behind facility greatly enhances disk performance at some price; turn the computer off at an improper time and you may lose data.
• Some databases are updated by a cron job; turning the computer off may leave the databases unuseable.