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Why were tanks developed during World War I?

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Kuwait: U.S. 1st Armored Division M1A1 Abrams tanks — tank M1A1 Abrams main battle tanks of the U.S. 1st Armored Division moving across the desert in northern Kuwait during the Persian Gulf War, February 1991.

Tanks were developed during World War I primarily to overcome the tactical stalemate of trench warfare and the overwhelming power of the defense.12

As the war solidified into continuous trench systems protected by barbed wire and machine guns, traditional infantry attacks resulted in catastrophic casualties. Armoured cars, which were used early in the war, proved ineffective in the broken, muddy terrain of the front lines. Consequently, military leaders sought a new type of tracked, armoured vehicle capable of:12

Moving off-road over uneven ground.1
Crossing wide trenches.12
Crushing or neutralizing enemy strongpoints.2
Providing mobile cover for advancing infantry.2

The first tanks, such as the British "Little Willie," were essentially motorized versions of ancient siege machines designed to break the deadlock of the Western Front. While early models were slow and had limited range, they were successfully employed to breach enemy lines, most notably at the Battle of Cambrai in 1917.12