The circuit board is made. Plain board with traces both sides. Then the board is lowered into a tank and the holes where the legs of components go are plated. This assures continuity of one side to the other for the traces if needed.
If the plating through the hole is damaged, just soldering a component in sometimes isn't enough. Like replacing large capacitors on a driver board. Often more damage is done than goo if you aren't skilled in board repair.
When I solder something into a board. I take a meter and check continuity of one of the legs of the component to a spot away from the repair area, along the same trace. To be sure continuity is good through board and across board.
One leg of the transistor open ( lost continuity ) can blow the transistor. I discovered this with a LOTR.
LTG : )