Please look at this image(see the soldered/un-soldered dotted-shapes which are identified by arrows).
This circuit belongs to a laptop adaptor.
What are these soldered/un-soldered dotted-shapes?
These act as test points. As pointed out by @oldfart, these could help facilitate bed-of-nails test. It looks similar to how it sounds:
This is important in case of mass manufactured items. In this case, before you label it as working, you might want to run some tests. For ex - is the output voltage correct, is the peak voltage appearing on mosfet is under limits, is the feedback signal correct etc. These can be either measured manually using a multimeter or by bed of nails test. Manual process will be time consuming. However you can make a jig out of spring loaded pins which will make contact to all traces of interest and then you can perform all measurements using a program within a second or so. You just hold the pcb in place and press a button. If all tests passed, you get a green signal otherwise you send it back to assembly line for repairs (and some low wage worker gets scolded upon by his manager).
As I can see, each trace just has one of them. why?
Answered above.
Why aren't they connected to ̶t̶h̶e̶ ̶ ̶"̶c̶a̶r̶g̶o̶ ̶c̶u̶l̶t̶s̶"̶? those soldered traces?
Most of these are electrically connected. However in some cases, you might see some unconnected dots. They are called fiducial marks (pointed out by @Hearth in comments). These help the pick and place machine align the component correctly while placing on the pads. It looks like this:
There are two fiducials on the PCB image that you shared:
Does this question have something related to this question.
Yes, old question explains the cargo cult joke. :)
