While the modern world of Wi-Fi has taken a lot of the wiring out of modern buildings, electricity is still needed throughout. Architects are aware of the needs of their clients, and they draw up electrical plans that will ensure it can be accessed most efficiently. The tradesmen who will wire the building will need to cut through joists, walls, floors and ceilings to place their wires, and they will also need to connect them through the main electrical panel.
Knowing how much wire they need and where is an important part of running the wires for any structure, so being able to refer to plans can be very helpful. It gives them an opportunity to get their measurements and placements right the first time, so they do not have to pull out their work and start over when a wire in the wrong area is discovered by a supervisor or inspector.