For one, the solder connections are more stable, and for another, well, they look so nice that you want to show them off. There are several ways to make boards including etching, direct plating and copper removal. For the hobbyist, etching is probably the easiest and most cost effective.
Etching is the process of chemically removing the unwanted copper from a plated board.
You must put a mask or resist on the portions of the copper that you want to remain after the etch. These portions that remain on the board are the traces that carry electrical current between devices. This article will discuss an alternative approach to placing the circuit pattern on the copper clad board. One way to put a pattern on the board is the direct draw approach. A resist pen is used to draw the circuit onto the copper surface.