In a multiuser environment like Linux, other users can log you out of your desktop session or access files on your system if you do not protect your work. By locking the screen, you can ensure that no one accesses your system when you are not at your computer.
You can also protect your work by changing your login password on a regular basis. This reduces the chances of someone logging on using your userid and gaining access to your files.
You can also set permissions for directories and files on your computer to ensure that they are not accidentally changed or destroyed. For information about setting permissions, see "Setting permissions and ownerships."