IBM has prepared a series of articles on understanding UNIX. One article I particularly liked is the one on understanding UNIX processes. While it is pretty easy to work with multiple applications at the same time in a graphic interface, it is far less intuitive on how to do this in a command line environment.
Being able to control processes can be very useful. For example, you start a very long task, such as searching a huge hard drive, and you want to do something else as the window scrolls by with thousands of lines of results. This can be done. Or, let's say that an application is slowing down your server more than you expected and you want to stop it for a few hours. This can be done, as well. Learning these commands will make your time on a UNIX/Linux server a much more productive one. These commands will also work within OS X given that OS X is a UNIX system.