Once, it is said, Shiva summoned His two Sons, Ganesha and Kartikeya-Murugan, and declared to them that whoever should complete three circumambulations of the world first will forever be declared the wisest of all. Kartikeya, whose mount is the peacock, immediately got on His vehicle and began to fly around the world. Ganesha, the Wise, whose mount was the mouse, knew that He could not possibly compete with Kartikeya. So, Ganesha used His wisdom, got on His vehicle, and circumambulated Shiva-Parvati instead. Despite Kartikeya's speed, Ganesha won the contest and was declared the wisest of all.
The above, of course, is only a story. Many different versions of this story exist, but the point is always the same. Stories such as these are created to illustrate important theological principles. The point is not to say that Ganesha is wise or Kartikeya is not. The point is that the wise always realize, as Ganesha demonstrates in the story, that the entire world, the whole of existence is a manifestation of Shiva-Shakti. Knowing Shiva-Shakti, there is nothing more to know.
We worship Lord Ganapati to bring us to that realization.
Aum Ganeshaya Namah.