Not to be too picky but it's the dif mount that has been replaced, the housing is the black roundish bit in the middle that rotates
The only reason to change the mounts out is damage to the original (cross threaded screws is the most likley), because it's lighter as Oli suggested (though relative weight isn't all that great a diference tbh) or, and this is most likely in my book, because it's shiny and a hop-up.. and god knows that hop-ups are, by definition, better than standard parts... no.. I'm not a cynic at all!
Having said that, if you are super lucky, it's possible to get upgraded dif cases.. alloy ones.. the stock ones are made of plastic and if you put too much torque through them the spider gear shafts can eat the insides...