Why is this PCB designed so weirdly?
Via stitching is used for impedance and noise control.  It lowers the resistance and inductance between two parallel planes.  The three contact areas do not have plated-through holes, so in those areas it probably is for current sharing between the top and bottom layer contact areas.

You also can use via stitching to create pseudo-coaxial cable around a trace.  For example, layers 1 and 3 are GND, and layer 2 has a trace for a delicate signal.  Rows of vias along both sides of the trace create a 3-d GND shield around the trace.  The closer the vias, the better the high frequency performance.

Via stitching around the edge of a board can prevent noise or signal radiation outward through the edge of a board.  This can work well in TEMPEST applications.