three theories:

There are many theories:
theory 1: a depression to collect sediment or residual oil when changing. this  depression keeps stuff from falling of flowing into the engine when you take the filter off.

theory 2: it's just something from the casting/machining i.e. to make it so a machine can handle the part or something like that. 

theory 3: subaru's future high end car has a sensor inside this depression so that it can detect oil leak :]