The metal gear housing protects from dirt and dust, as well as keeping the system more durable. (geekwrapped.com)
The heavy duty metal gear housing gives it a lot of ruggedness and helps it stand up to wear and tear. (geekwrapped.com)
However, an extra shaft from this differential also drives a bevel gear housed in the transfer case that permanently turns a driveshaft for the rear wheels (i.e. there is no longer a «center» differential). (en.wikipedia.org)