Pencil is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil is free and open source.
Pencil is not intended to mimic web-oriented vector-based animation software such as Flash. Its main purpose is to make traditional animation. Neither does it try to rival commercial software targeting the professional animation sector. Pencil is intended to be a simple program that could be used by anyone to make 2D animation.
So the main question here is not why to even choose Pencil.
The real question is : “Do you want to make traditional animation ?”
If your passions are related to this field , than you’re on the right track.