The bearings and rear Axle Housing are key components of the Rear Axle assembly.Therefore, they are designed to support and align the Differential assembly and the drive Axles.
Firstly, Notice that the bearings and Axle Housing are large, heavy-duty parts. This is to ensure they will stand up under hard usage.
Gaskets are used at housing interfaces, such as between the differential cover and the housing. This is to provide a tight seal from the outside.