If you hide (command-H) an application when you're done using it instead of quitting it, you avoid the delay which occurs when you need to use that application again. There's almost no penalty for doing this, except perhaps a slightly overcrowded dock.
Now, for example, when you want to view a help page, you'll get to view it immediately, rather than having to wait for the Help application to launch.
[Editor's note: There's almost no penalty to a point. If you start to hear lots and lots of hard drive activity, you're probably pushing the limit. The more RAM you have, obviously, the more apps you can leave open.]

