Why was the telegraph used in the civil war?

The electric telegraph was the most advanced technology of the time. It not only tied together major cities and organizations but, also the continents. Prior to the war a transatlantic cable was laid, although it did not work for very long it was an important experiment in communications.

When the armies took to the field, they brought telegraphic support to the commanders via temporary telegraph lines that could be linked to national lines. This was remarkable in that it was probably the first time that lower level tactical and national-level strategic communications were available to a field commander located far from the capital. It allowed the coordination of forces, transportation, logistics, etc., and changed the nature of warfare.

The American Civil War marked a new age of military communication as new organizations, most notably the Signal Corps and the Military Telegraph, provided dedicated corps of military and civilian communications specialists who employed the most advanced technologies of the time.