When you are making some prototype PCBs it is quite simple until you go double sided, now you need to potentially solder through hole components on both sides, stick small wires in each via and solder both sides or insert small rivets in the holes. DIYyouwhere is playing around with a method where they attach a brush to the drill so that they can electrically detect when the metal drill bit drills through the PCB and hits the bottom layer of the copper. As soon as it detects contact the bit is retracted. They now use a syringe of conductive ink to link the copper layers together by squirting some ink in each hole.