Although the radio (or as it was initially called, the wireless) was invented at the end of the 19th century, it found its first significant use during WWI. Radio transmission was still very unreliable, and on the battlefields, soldiers preferred other forms of communication. Yet its wartime employment ushered in the larger peacetime use.

More than any previous war, WWI greatly relied on communication. Armies soon realised that improved electric communication of greater capacity between larger units and within regiments was desirable.

When the war started, communication was still made prominently over lines, like telephone and telegraph. Field telephone and switchboard were greatly implemented. 
Soldiers on all sides lay miles and miles of cable every day, using wagons pulled by horses. They created an intricate network of buried cables and wires operating in the elaborate trench system, which eventually reached the forwardmost outposts. Pole lines with many crossarms and circuits rose in the rear of all armies, which main arteries ran from the rear to the forward trenches. Lateral cable routes roughly parallel to the front intersected them, creating a gridwork of deeply buried cables, with underground junction boxes and test points every few hundred yards. 
It was a complex system – which had its disadvantages.
Cables were constantly damaged by bombing, shell fire, and by the very movement of the troops. And if this was not enough, soon it was discovered that intercepting communications over the wire wasn’t particularly difficult.

But there were many different alternatives (including light signals, homing pigeons, dogs and runners). Radio was one of these.

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Wartime radio communications 

Radio transmission had existed since the late 19th century, but the wide and capillary use of telegraph had for a time suggested that a wireless system was unnecessary. 
Yet, in some fields – like the navy – radio transmission turned out to be handier. At the beginning of the 20th century, ships depended on the radio to communicate with other ships and with onshore radio stations. Since it depended greatly on the weather condition, the signal could be unstable and voice communication unclear, but it was operational. 

Radio sets at the opening of WWI were primitive, had a very short range, and required an average of three men to transport. Its well-known dependence on good weather conditions had initially limited its use. 
But throughout the war, many innovations bettered the transmission and receptions of messages. Smaller and more portable sets powered by batteries and using small, inconspicuous aerials eventually came into use. A partial solution was found in the final years of the war, when what would afterwards be known as supereterodyne receiver was invented. It was a way to pick up distance radio signals by tuning in on the radio waves. This made radio transition a lot clearer and more reliable. Soon, radio equipment came to be used in all unit headquarters, including battalions. 

Despite all of this, radio communication still presented many difficulties. It was easy to intercept, therefore messages needed to be cryptographed or encoded. Voice communication, though possible, remained unclear and sometimes unintelligible. The equipment itself was always quite experimental. 
For all these reasons, radio communication in WWI was always auxiliary to the wire system and mostly used in an emergency when the wire lines were cut and unusable. 

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RESOURCES

Army radio communication in the Great War by Keith R Thrower (pdf)

Britannica – Military communication
BBC News – World War One: How radio crackled into life in conflict
Spectrum – World War I: the war of inventors
The United States World War One Centennial Commission – Spreading the American Spirit through the Airwaves

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