Created "calculus", in parallel with Newton. This proves that this direction of research was not accidental, but was dictated by the needs of productive forces of its times, specifically the need to develop mechanics as a science, and hence the Industrial revolution, a century later.
Leibniz was not only a mathematician, but a multi-dimensional person. He created a philosophy of a monad, and invented various machines, some of which are early ancestors of a computer. Wikipedia: In England, "He met with the Royal Society where he demonstrated a calculating machine that he had designed and had been building since 1670. The machine was able to execute all four basic operations (adding, subtracting, multiplying, and dividing), and the society quickly made him an external member."