I'm not 100% in that image specifically.
A lot of the time, laptops will have a "range" of models. The chassis (for the most part) is the same....yet certain areas are not populated, so there's simply a "cover" in place to stop it from just being a gaping hole on the side.
Card readers & ExpressCard slots would be some of the 'standard' things that could likely fill that slot (depending on the age of the laptop).