Possible reasons for placing a man who is in the act of excreting waste in a scene which is widely considered holy include:

* Tradition.

* Perceived humor.

* Finding the Caganer is a fun game, especially for children.

* The Caganer, by creating feces, is fertilizing the Earth. However, this is probably an a posteriori explanation, and few cite this reason for including the Caganer in the Nativity scene.

* The Caganer represents the equality of all people: regardless of status, race, or gender, everyone defecates.

* Increased naturalism of an otherwise archetypal (thus idealised) story, so that it is more believable, taken literally and seriously.

* The idea that God will manifest himself when he is ready, without regard for whether we human beings are ready or not.

* The caganer reinforces that the infant Jesus is God in human form, with all that being human implies.

via Caganer – Wikipedia, the free encyclopedia.