Why:
Because I got my last two N9s from China and they often have things from a previous owner left over. Also I like to have the Alps maps ready at start. Flashing EMMC clears all data like images,stored maps, music and so on and places some standard music and maps instead.

How often:
Once when I first set the device up.

btw: I can thoroughly recommend this China store if anybody wants to get an as new N9.