Say you use Time Machine on Leopard to back up all of your important work. Then say you just finished/saved a bunch of work and you want to shut down your computer. But you want to make sure your work is saved. What to do? It so happens that you can force Time Machine to perform a backup. Just right-click on the Time Machine icon in the Dock and you'll get a menu with an option to "Back Up Now". Now you can go home with your mind at ease :)