You won't need a Barlow often. A couple of uses come to mind, however: first, a Barlow lets you use long-focal-length eyepieces with their superior eye relief (a 6mm Plossl is hard to use). However, most of the time you won't want to go above 350x or so, since the atmosphere won't let you.
Another use for the Barlow is for prime-focus photography, making the image of a bright object twice the size on your CCD. You may go a lifetime without needing a Barlow. But if you do get one, don't get a cheap one.