Solder mask free regions on PCB
As I have seen mentioned in the comments, this could be used to apply solder to conduct higher currents.

However, I don't believe that is the intended purpose in the photos provided.  Notice how the exposed traces surround specific sections of the board completely, and don't appear to be connected to anything except likely to ground.  There are also a large number of stitching vias. This would be connected to a metal can or some other shielding.