Weapons of war.
Leonardo's life’s work was a search for the meaning of creation itself and his many varied studies are a testament to his inquiring mind. His work on war machines seems to contradict his respect for the natural world. However, Leonardo was aware of the dangers presented by science and the impact that the exploitation of natural resources may have on the earth.
He was a man of his time and the need for military engineers provided him with employment, travel opportunities, and the chance to continue his scientific work unhindered.
Renaissance Italy was a collection of independent city-states who became engaged in incessant warfare with each other. This provided a market for the technically advanced weapons needed to gain a military advantage over the enemy.
In short:- a Renaissance arms war!
Leonardo had studied and absorbed both classical and contemporary works on military engineering. In his famous letter to Ludovico il Moro Sforza, Duke of Milan, he listed nine categories of military engineering among the many skills that he was proficient in.
His items included; new weaponry, bridging, bombarding machines and trench draining.