An electrical schematic diagram provides electrical engineers with an idea of how a circuit layout should look when it is completed, and it also aids in planning repairs. For example, looking at an electrical schematic diagram for a circuit board indicates where wires connect between components of the system.
On a larger scale, electrical schematics help plans for wiring buildings. For safety, circuits can only carry a given amount of current. For example, if a light circuit carries 10 amperes, that information appears on the schematics. Referring to the diagram during construction reduces the likelihood of potentially dangerous wiring mistakes.