Short answers, pulled from the story.
The British Gun Carrier Mark I became the first tracked self-proposed gun fielded in 1917 during World War I. It was based on the British Mark I tank and carried a heavy field gun that could be fired from the vehicle or removed for standard setup.
Hauptmann Alfred Becker created the first battery of self-propelled artillery by mounting 10.5 cm leFH 16 howitzers on captured British Vickers Mk.VI light tanks. His Geschützwagen Mk VI 736 (e) served as the forerunner to German tracked field guns like the Wespe and Hummel.
The key advantage of self-propelled over towed artillery lies in the ability to bring weapons into action much faster. Self-propelled units can stop at a chosen spot and begin firing almost immediately before moving on, whereas towed systems must stop, unlimber, and set up guns before they can be used.
Italy entered service with the Italian 102/35 su SPA 9000 in September 1915 as the first wheeled heavy self-propelled gun. More than 100 units were produced and fought in every major battle on the Italian front during World War I.
A modern battery of six guns can fire 43 kg projectiles at a burst speed of four rounds per minute. This configuration delivers over a tonne of ordnance per minute for up to four minutes with immense weight of fire.