Sometimes it may be more efficient to use pencil lines instead of brush strokes when you are designing your characters. Since a pencil line has a single centre vector line, it allows you to change the thickness of the line uniformly as you are working by selecting the line and changing the maximum size of your pencil.
However, pencil lines use more memory than brush lines, if your artwork is more or less complete it may be advantageous to convert them to Brush lines to save some more memory. Converting pencil lines to brush lines will make the software much more responsive allowing you to focus on animating. Brush lines also allow for more artistic variation, as you can create varying thick and thin lines, as well as tapers. So style and aethetics are also other factors to consider.
Mac OS X (all)