Why are there unconnected exposed pads on PCBs
They are Fiducial Marks and are often used for visual alignment of automated processes on PCB manufacturing and assembly.

These automated processes can vary from pick and place machines to visual inspection and flying probes.

To account for possible rotation of the board there will be at least 2 points on a PCB. In fine pitch projects there may be more fiducials near critical components. (ty Sean for pointing that out)