Excuse the vagueness of the question; I am a beginner.

I currently put all my video onto one track. It dawned on me that I do not have to do this, but I can not think of any good reason not to. When do you use multiple video tracks?

The best answer I can give: When it is necessary to do so.

For example, if you want to add a title, you will need another video track. If you want to create a picture-in-picture effect, you will need another video track. These are just two of many reasons.

Sometimes it can be handy if you have lengthy segments of two
or more video's / scenes where you cut a lot between those. Then
you don't have to rearrange everything on one track all the time.

composites.. overlays, titles.. masks... err... effects.. hmm..

Also see this thread (http://www.dvinfo.net/conf/showthread.php?s=&threadid=23215)