After some research both in Catalan web files and Spanish ones we can conclude that noone has a clue about what those crappers (henceforth referred as 'caganers') do represent and why do they start appearing in Catalan Nativity scenes, henceforth referred as 'pessebres'.
It is clear though that they started appearing during the late XVIIIth century, in a moment where the traditional Catalan society started to swift from noblemen and plebs to a more varying range which included people who had made vast fortunes with trade of goods and wanted to show off.
So, in essence, we can conclude that the 'caganer' is simply implemented into 'pessebres' as a way of showing the ancient nobility that other people could afford to make one for their personal enjoyment (In a quite sick but nonetheless expressive way, might we be allowed to express our opinion).
There are some amongst those who have studied the appearance on the Catalan 'pessebre' which have different theories about its meaning and also about why this singular figure has survived in this 'globalised' era, but this would probably give us material for discussion for some two pages and take me some half an hour to ellaborate. (And as I must open the bookshop right now I will have to leave it here)