It used to be easy to figure out whether or not a wire belonged in a wall, and many electricians found they could even use some of the old wiring to tie into new areas. Building codes have been upgraded now, and tearing out the old wiring has become mandatory in some places. Getting rid of this excess baggage gives those in homes the knowledge electrical fires will be less likely, and they can also skip the tasteless look of bare wiring.
Some areas of any building will still need wiring, and adding new wires can be expensive. While it might seem possible to simply use the old wires, modern building codes often call for replacement. Putting two different types of wire together can cause arcs in the walls, and it could lead to a fire. Safety has become an important issue where any electricity is concerned, so those who choose to remodel any part of their home will often find they must remove all the wiring in the walls before they can finish their work.