How do you talk to someone on the other side of the planet?
At the dawn of civilization, one might dispatch messengers via horseback (or fast runner in the Western Hemisphere, horses being unknown until the Conquistadores came). That might take months or even years. Smoke signals and heliographs were a little better, but they still were limited to line of sight transmission.
The telegraph was a revolution. Now, messages could travel from point to point at the speed of light. A few decades later, telephones enabled live conversations at great distance. Radio broadcasts shrunk the world further, broadcasting messages wirelessly throughout the globe.
But neither the telephone nor radio are perfect solutions to the presented problem. With telephones, both parties need to be physically connected to each other. Radio is notoriously unreliable at great distances. Things are worse if you're in the military--neither phones nor the wireless are secure: wires can be tapped, and radio is broadcast in the clear.
What you really want is a tight-beam radio broadcast, one that could be directed at any recipient without need for wires. But for that, you'd need a series of repeating towers that provide service anywhere on Earth. That's a tall order. Not only is it expensive, but pesky oceans get in the way. You could get away with fewer towers if they were tall enough, but how do you construct a 100-mile high repeating tower?
(read the answer at Galactic Journey!)